Hologram screen and a method of producing the same

ABSTRACT

In a hologram screen for reproducing an image based on an output light obtained by scattering and diffusing an image light from an image projector, when a white light (image light) is projected on the hologram screen; and when a distance between two points at optional two points A and B on a surface of the hologram screen is given by 20 cm or less, and a value of the CIE 1976 chromaticity coordinate (u′, v′) at the point A is given by (u′ A , v′ A ) and the value of the CIE 1976 chromaticity coordinate (u′, v′) at the point B is given by (u′ B , v′ B ); the output light perpendicularly from the surface of the hologram screen has a color distribution in which a color difference Δu′v′ between two points A and B is derived from the following formula (1) and given by 0.06 or less, where, the formula (1) is expressed by,  
     Δ u′v ′=[( u′   A   −u′   B ) 2 +( v′   A   −v   B ) 2 ] 1/2   (1)  
     Further, the output light, which is output to a viewpoint defined by the following formula (2) from the surface of the hologram screen, has the color distribution in which a color difference Δu′v′ between two points A and B is derived from the above formula (1) and given by 0.01 or less, where, the formula (2) is expressed by,  
       H /2 L =0.1  (2)  
     in the formula (2), L is a distance between the viewpoint and a center of the hologram screen, and H is a length of the hologram screen in the direction of the height.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention relates to a hologram screen and a methodof producing the same. In particular, a first aspect of the presentinvention relates to a hologram screen for reproducing an image based onan output light obtained by scattering and diffusing an image lightwhich is input from an image projector, and a producing method of thesame; a second aspect of the present invention relates to a producingmethod of a hologram element which can be used as a display apparatusfor displaying the image, such as a still image and an animation, byirradiating the image light thereto; a third aspect of the presentinvention relates to a hologram screen and a producing method of thesame; and a fourth aspect of the present invention relates to a hologramscreen for reproducing an image based on an output light obtained byscattering and diffusing an image light.

[0003] 2. Description of the Related Art

[0004] A hologram screen, which is formed by a hologram element and atransparent glass, has been widely known in the field of imageprojecting techniques. A hologram element is of basically two types,i.e., a light-transmitting type and a light-reflecting type and theposition of an observer who observes the hologram screen is different ineach type.

[0005] As representative problems to be solved in a conventionalhologram screen, there are the quality of the image, such as unevennessof color and brightness on the hologram screen, cloudiness and a partialnon-transparent state on the hologram screen, and the difficulty ofpreparing very large hologram screen. These problems will be explainedin detail with reference to the attached drawings described below.

SUMMARY OF THE INVENTION

[0006] The first object of the present invention is to provide ahologram screen, which has no unevenness of color on the reproducedimage, and a method of producing the screen

[0007] The second object of the present invention is to provide a methodof producing a hologram screen in which an observer can observe auniformly bright image without change of brightness and color to a broadextent on the hologram screen.

[0008] The third object of the present invention is to provide ahologram screen, which has good reappearance of the reproduced image onthe hologram screen and which can realize low cost and easy productionof the hologram screen, and a method of producing the screen.

[0009] The fourth object of the present invention is to provide ahologram screen which has good transparency and no cloudy state so thatthe observer clearly and easily observe the back of the hologram screen.

[0010] In accordance with a first aspect of the present invention, thereis provided a hologram screen, and a method of producing the same, forreproducing an image based on an output light obtained by scattering anddiffusing an image light from an image projector, characterized in that;when a white light is used as the image light and projected on thehologram screen; and when a distance between two points at optional twopoints A and B on a surface of the hologram screen is given by 20 cm orless, and a value of the CIE 1976 chromaticity coordinate (u′, v′) atthe point A is given by (u′_(A), v′_(A)) and the value of the CIE 1976chromaticity coordinate (u′, v′) at the point B is given by (u′_(B),v′_(B)); the output light, which is output perpendicularly from thesurface of the hologram screen, has a color distribution in which acolor difference Δu′v′ between two points A and B is derived from thefollowing formula (1) and given by 0.06 or less, where, the formula (1)is expressed by,

Δu′v′32 [(u′ _(A) −u′ _(B))+(v′ _(A) −v′ _(B))²]^(1/2)   (1)

[0011] Further, the output light, which is output to a viewpoint definedby the following formula (2) from the surface of the hologram screen,has a color distribution in which a color difference Δu′v′ between twopoints A and B is derived from the above formula (1) and given by 0.01or less, where, the formula (2) is expressed by,

H/2L=0.1  (2)

[0012] in the formula (2), L is a distance between the viewpoint and acenter of the hologram screen, and H is a length of the hologram screenat the direction of the height.

[0013] In a preferred embodiment, a spectral characteristic of theoutput light, which is output perpendicularly from the surface of thehologram screen, has a characteristic in which either a differencebetween peak wavelengths at the whole of the surface of the hologramscreen is given by 120 nm or less, or a difference between halfbandwidth is given by 100 nm or less.

[0014] In another preferred embodiment, when producing the hologramscreen defined in claim 1 or 2 by irradiating the object light and thereference light onto a photosensitive material and by exposing thephotosensitive material, the photosensitive material has a thicknessdistribution which is inclined to an incident direction of the referencelight.

[0015] In still another preferred embodiment, when an incident angle ofthe reference light to the photosensitive material becomes larger, thethickness distribution of the photosensitive material becomes thicker.

[0016] In still another preferred embodiment, when the thickness at acenter of the photosensitive material is T₀, and when thicknessdifference of both ends of the photosensitive material is ΔT, thefollowing relationship, i.e., ΔT≦0.5 T₀, is defined.

[0017] In still another preferred embodiment, the thickness distributionof the photosensitive material is defined by the following formula (3),i.e.,

T=a(R ₀ −R)+T ₀, and a=b·θ ₀ ^(−0.9)  (3)

[0018] where, T is a thickness at an optional point on thephotosensitive material, T₀ is a thickness at the center of thephotosensitive material, R is an incident distance of the referencelight at the optional point on the photosensitive material, R₀ is theincident distance of the reference light at the center on thephotosensitive material, θ₀ is an incident angle of the reference lightat the center of the photosensitive material, and “b” is a coefficientdetermined by the thickness of the photosensitive material and given by0<b<1.

[0019] In still another preferred embodiment, when producing thehologram screen defined in claim 1 or 2 by irradiating the object lightand the reference light onto a photosensitive material and by exposingthe photosensitive material, ultraviolet light is previously irradiatedonto the photosensitive material so as to have an energy distributionwhich is inclined to an incident direction of the reference light.

[0020] In still another preferred embodiment, the energy distribution ofthe ultraviolet is given by the following formula (4), i.e.,

0.8E≦Euv≦1.2E, and E=0.01(R−R ₀)+E ₀  (4)

[0021] where, R is the incident distance of the reference light at theoptional point on the photosensitive material, Euv is an amount ofenergy of the ultraviolet irradiated to the optional point on thephotosensitive material, R₀ is the incident distance of the referencelight at the center of the photosensitive material, and E₀ is the amountof energy of the ultraviolet irradiated to the center of thephotosensitive material.

[0022] In still another preferred embodiment, when producing thehologram screen defined in claim 1 or 2 by irradiating the object lightand the reference light onto a photosensitive material and by exposingthe photosensitive material, an incident distance of the reference lightat an optional point of the reference light is shorter than an incidentdistance of an image light which is irradiated from an image projectorwhen reproducing the image on the hologram screen.

[0023] In still another preferred embodiment, when producing thehologram screen defined in claim 1 or 2 by irradiating the object lightand the reference light onto a photosensitive material and by exposingthe photosensitive material, an intensity ratio of the reference lightand the object light is defined by a relationship of I_(R)/I_(O)≦10, onthe whole of the surface of the photosensitive material, where, I_(R) isthe intensity of the reference light, and I_(O) is the intensity of theobject light.

[0024] In accordance with a second aspect of the present invention,there is provided a method for producing a hologram screen formed byinterfering of a reference light and an object light transmitted througha light diffusion body, and by recording the light diffusion body on aphotosensitive material, characterized in that; a mirror is arrangedapproximately perpendicular to the light diffusion body; thephotosensitive material and the light diffusion body are rotated by thesame angle θ around an axis which is intersected perpendicularly to acenter of the photosensitive material and used as a rotational center;and a polarized direction of a laser light is defined by eitherP-polarization or S-polarization which is inclined by the angle in therange of θ−5 to θ+5.

[0025] In accordance with a third aspect of the present invention, thereis provided a hologram screen for reproducing an image based on anoutput light obtained by scattering and diffusing an image light from animage projector, characterized in that; a half bandwidth of a spectralcharacteristic of the hologram screen is given by 100 nm or more; adiffusion light which is obtained by a light diffusion body having alarge light diffusion angle is used as an object light; a non-diffusionlight is used as a reference light; and the object light and thereference light are irradiated on a photosensitive material in order toform an interference fringe so that a hologram element is produced.

[0026] In a preferred embodiment, the light diffusion angle of thehologram screen is defined by an angle in which the light diffusionangle of the hologram screen is 100 or more.

[0027] In another preferred embodiment, a half bandwidth of a spectralcharacteristic of the hologram screen is given by 100 nm or more; adiffusion light which is obtained by a light diffusion body is used asan object light; a non-diffusion light is used as a reference light; andthe object light and the reference light are irradiated on aphotosensitive material having the thickness of 1 to 20 μm in order toform an interference fringe so that a hologram element is produced.

[0028] In still another preferred embodiment, a peak wavelength of thehologram screen is either 525 nm or less, or 585 nm or more; a halfbandwidth of a spectral characteristic of the hologram screen is 100 nmor more; a diffusion light which is obtained by a light diffusion bodyis used as an object light; a non-diffusion light is used as a referencelight; the object light and the reference light are irradiated on aphotosensitive material in order to form an interference fringe; and arefractive index of the photosensitive material is adjusted so that ahologram element is produced.

[0029] In still another preferred embodiment, the diffusion light whichis obtained by the light diffusion body is used as the object light; thenon-diffusion light is used as the reference light; the object light andthe reference light are irradiated on the photosensitive material inorder to form the interference fringe; and a heat treatment is performedfor the photosensitive material in the extent of 80° to 150° C. so thata hologram element is produced.

[0030] In still another preferred embodiment, the diffusion light whichis obtained by the light diffusion body is used as the object light; thenon-diffusion light is used as the reference light; a sum of an exposureintensity of the object light and the exposure intensity of thereference light is defined in the extent of 0.02 to 50 mW/cm²; theobject light and the reference light are irradiated on thephotosensitive material in order to form the interference fringe so thata hologram element is produced.

[0031] In still another preferred embodiment, the diffusion light whichis obtained by the light diffusion body is used as the object light; thenon-diffusion light is used as the reference light; an intensity ratio(R/O) of the intensity (O) of the object light and the intensity (R) ofthe reference right is defined by 0.1 to 30; and the object light andthe reference light are irradiated on the photosensitive material inorder to form the interference fringe so that a hologram element isproduced.

[0032] In still another preferred embodiment, a peak wavelength of thehologram screen is either 525 nm or less, or 585 nm or more; a diffusionlight which is obtained by a light diffusion body is used as an objectlight; a non-diffusion light is used as a reference light; the objectlight and the reference light are irradiated on a photosensitivematerial in order to form an interference fringe; and a thickness of ofthe photosensitive material is adjusted so that a hologram element isproduced.

[0033] In still another preferred embodiment, a peak wavelength of thehologram screen is either 525 nm or less, or 585 nm or more; a diffusionlight which is obtained by a light diffusion body is used as an objectlight; a non-diffusion light is used as a reference light; an incidentangle θ_(r) of the reference light to a photosensitive material isdifferent from an incident angle θ_(e) of the image light to thehologram screen; and the object light and the reference light eachhaving the above different angle are irradiated on the photosensitivematerial in order to form an interference fringe so that a hologramelement is produced.

[0034] In still another preferred embodiment, an amount of anglecorrection which indicates a difference between the incident angle θ_(r)and the incident angle θ_(e) is defined by the extent of −5° to +50.

[0035] In still another preferred embodiment, a half bandwidth of aspectrum characteristic of the hologram screen is given by 100 nm ormore; a diffusion light which is obtained by a light diffusion body isused as an object light; a non-diffusion light is used as a referencelight; and a plurality of object lights each having different angle areirradiated on a photosensitive material in order to form an interferencefringe so that a hologram element is produced.

[0036] In accordance with a third aspect of the present invention, thereis provided a hologram screen for reproducing an image based on anoutput light obtained by scattering and diffusing an image light from animage projector, characterized in that, a haze ratio is given by 5 to60%.

[0037] In a preferred embodiment, a screen gain of the hologram screenis given by 0.3 or more.

[0038] In another preferred embodiment, an intensity ratio E_(R)/E_(O)of the intensity E_(O) of the object light and the intensity E_(R) ofthe reference light is changed in accordance with a scattering angle ofthe light diffusion body.

[0039] In still another preferred embodiment, when the scattering angleof the light diffusion body is set to a large angle, the intensity ratioE_(R)/E_(O) is set to a small value.

BRIEF DESCRIPTION OF THE DRAWINGS

[0040]FIG. 1 is a view for explaining use state of a hologram screenaccording to a first embodiment of a first aspect of the presentinvention;

[0041]FIG. 2 is a view for explaining optional two points A and B on thesurface of the hologram screen shown in FIG. 1:

[0042]FIG. 3 is an explanatory view for checking a spectralcharacteristic of an output light which is output perpendicularly fromeach of points (1) to (3) on the hologram screen in the firstembodiment;

[0043]FIG. 4 is a view for explaining a formula which derives X in thefirst embodiment;

[0044]FIG. 5 is an explanatory view for checking unevenness of color inthe first embodiment;

[0045]FIG. 6 is a graph for explaining a result of test of unevenness ofcolor in the first embodiment;

[0046]FIG. 7 is an explanatory view for checking the spectralcharacteristic of the output light which is output downward with anangle “g” from each of points (1) to (3) on the hologram screen in thefirst embodiment;

[0047]FIG. 8 is an explanatory view for checking the spectralcharacteristic of the output light toward the view point in the secondembodiment;

[0048]FIG. 9 is an explanatory view for checking the spectralcharacteristic of the output light which is output downward with theangle “g” from the perpendicular direction to the view point on thesurface of the hologram screen in the second embodiment;

[0049]FIG. 10 is an explanatory view for an exposure optical systemaccording to a third embodiment in the first aspect of the presentinvention;

[0050]FIG. 11 is an explanatory view for an essential parts of theexposure optical system in the third embodiment;

[0051]FIG. 12 is a graph for explaining thickness distribution of thephotosensitive material of a sample 1 and comparing sample C1 in thethird embodiment;

[0052]FIG. 13 is a graph for explaining the spectral characteristic ofthe hologram screen for the sample 1 in the third embodiment;

[0053]FIG. 14 is a graph for explaining the spectral characteristic ofthe hologram screen for the comparing sample 1 in the third embodiment;

[0054]FIG. 15 is an explanatory view for irradiation of an ultravioletto the photosensitive material according to a fourth embodiment;

[0055]FIG. 16 is a graph for explaining an amount of the ultraviolet tothe photosensitive material;

[0056]FIG. 17 is a graph for explaining an amount of absorption of thelight having the wavelength of 514 nm on the photosensitive materialafter irradiation of the ultraviolet in the fourth embodiment;

[0057]FIG. 18 is a view for explaining the relationship between theamount of absorption of the light (wavelength of 514 nm) and the amountof irradiation of the ultraviolet in the fourth embodiment;

[0058]FIG. 19 is a graph for explaining the relationship between thepeak wavelength and the amount of irradiation of the ultraviolet in thespectral characteristic of the hologram screen obtained by thephotosensitive material after irradiation of the ultraviolet in thefourth embodiment;

[0059]FIG. 20 is an explanatory view for reduction of irradiationdistance of the reference light according to a fifth embodiment;

[0060]FIG. 21 is an explanatory view for an incident angle θ_(x) of theobject light which is input from a point X1 of the light diffusion bodyto a point X2 of the photosensitive material;

[0061]FIG. 22 is an explanatory view for the incident angle θ_(y) of theimage light which is input to a point y on the hologram screen, and anoutput angle θ_(z) of of the output light which is diffracted y thehologram screen;

[0062]FIG. 23 is a graph for explaining the relationship between areduced amount of the incident distance of the reference light, and thepeak wavelength at the spectral characteristic of each hologram screenafter reduction of the incident distance;

[0063]FIG. 24 is a graph for the spectral characteristic of the hologramscreen having the reduced amount of −700 mm in the fifth embodiment;

[0064]FIG. 25 is a graph for explaining the relationship between thereduced amount of the incident distance of the reference light, and themaximum color difference on the hologram screen after reduction in thefifth embodiment;

[0065]FIG. 26 is an explanatory view for the essential parts of theexposure optical system having a convex lens which is arranged betweenan object lens and the photosensitive material in order to reduce theincident distance of the reference light according to a sixthembodiment;

[0066]FIG. 27 is an explanatory view for the essential parts of theexposure optical system having a concave lens which is arranged betweenan object lens and the photosensitive material in order to reduce theincident distance of the reference light in the sixth embodiment:

[0067]FIG. 28 is an explanatory view for configuration of the referencelight and the object light which are input to the photosensitivematerial;

[0068]FIG. 29 is a graph for explaining the relationship between theintensity I_(R)/I_(O) and the maximum color difference in the seventhembodiment:

[0069]FIG. 30 is an explanatory view for the exposure optical systemaccording to a first embodiment of the second aspect of the presentinvention;

[0070]FIG. 31 is a perspective view of the light diffusion body in thefirst embodiment;

[0071]FIG. 32 is an essential view of the light diffusion body in thefirst embodiment;

[0072]FIG. 33 is a plane view of the light diffusion body in the firstembodiment;

[0073]FIG. 34 is an explanatory view of the hologram screen in the firstembodiment;

[0074]FIG. 35 is an explanatory view of the display apparatus using thehologram screen in the first embodiment;

[0075]FIGS. 36A and 36B are explanatory views for explaining connectionmethod of the hologram screen in the first embodiment;

[0076]FIG. 37 is an explanatory view for position of the light diffusionbody and the reference light when exposing the photosensitive material;

[0077]FIG. 38 is a plane view for the photosensitive material, the lightdiffusion body and the reference light which are observed from thez-axis direction of FIG. 37;

[0078]FIG. 39 is a plane view for explaining rotated state of thephotosensitive material of FIG. 38, the light diffusion body and thereference light;

[0079]FIG. 40 is a plane view for the relationship between the lightdiffusion unit and the polarized direction;

[0080]FIG. 41 is a plane view for rotated state of the light diffusionunit of FIG. 40;

[0081]FIG. 42 is an explanatory view for the polarized direction of theobject light Oa transmitted through the light diffusion unit, and theobject light Ob reflected by the mirror;

[0082]FIG. 43 is an explanatory view for the relationship between theobject light Ob of FIG. 42 and the reference light at the opticalsystem;

[0083]FIG. 44 is an explanatory view for an observing method of thehologram screen in the second embodiment of the second aspect of thepresent invention;

[0084]FIG. 45 is an explanatory view for a measuring method of thehologram screen in the second embodiment;

[0085]FIG. 46 is a graph for the relationship between measuring pointsand brightness on the hologram screen in the second embodiment;

[0086]FIG. 47 is a graph for the relationship between measuring pointsand chromaticity coordinate (u′) on the hologram screen in the secondembodiment;

[0087]FIG. 48 is a graph for the relationship between measuring pointsand chromaticity coordinate (v′) on the hologram screen in the secondembodiment;

[0088]FIG. 49 is a graph for the relationship between measuring pointsand brightness on the hologram screen (compared example) in the secondembodiment;

[0089]FIG. 50 is a graph for the relationship between measuring pointsand chromaticity coordinate (u′) on the hologram screen (comparedexample) in the second embodiment;

[0090]FIG. 51 is a graph for the relationship between measuring pointsand chromaticity coordinate (v′) on the hologram screen (comparedexample) in the second embodiment;

[0091]FIG. 52 is an explanatory view for the exposure optical system asanother example;

[0092]FIG. 53 is an explanatory view for the essential parts of theexposure optical system according to a first embodiment of the thirdaspect of the present invention;

[0093]FIG. 54 is an explanatory view for the object light, the referencelight and the photosensitive material in the exposure optical system inthe first embodiment shown in FIG. 53;

[0094]FIG. 55 is an explanatory view for the exposure optical systemaccording to the first embodiment;

[0095]FIG. 56 is an explanatory view for the display apparatus using thehologram screen according to the first embodiment of the presentinvention;

[0096]FIG. 57 is a graph for the relationship among the number ofdouble-faced ground glass, the output angle from the light diffusionbody, and the intensity ratio in the first embodiment;

[0097]FIG. 58 is a graph for the relationship between the lightscattering angle of each light diffusion body and the half bandwidth ofthe hologram element in the first embodiment;

[0098]FIG. 59 is a graph for the relationship between the lightdiffusion angle of each light diffusion body and the light diffusionangle of the hologram element in the first embodiment;

[0099]FIG. 60 is an explanatory view for a measuring method of thespectrum of the hologram;

[0100]FIG. 61 is a graph for the relationship among the half bandwidth,the peak wavelength, and the peak efficiency in the spectrum of thehologram;

[0101]FIG. 62 is a graph for the relationship among the half bandwidth,an evaluation level, and the number of observers in the spectrum of thehologram;

[0102]FIG. 63 is a graph for the relationship between the half bandwidthand the evaluation level in the spectrum of the hologram;

[0103]FIG. 64 is a graph for the relationship between thickness of thephotosensitive material and the half bandwidth in the second embodimentof the second aspect of the present invention;

[0104]FIG. 65 is a graph for the relationship between the thickness ofthe photosensitive material and the peak efficiency in the secondembodiment;

[0105]FIG. 66 is a graph for the relationship between the intensity R/Oand the half bandwidth in the third embodiment;

[0106]FIG. 67 is a graph for the relationship between the intensity R/Oand the efficiency in the third embodiment;

[0107]FIG. 68 is a graph for the relationship between a spectralluminous efficiency and the wavelength in the fourth embodiment;

[0108]FIG. 69 is a graph for the relationship between the efficiency andthe wavelength in the spectrum of the hologram in the fourth embodiment;

[0109]FIG. 70 is a graph for the spectrum of the hologram when R/O is 2,and when the peak wavelength is 547 nm, in the fourth embodiment;

[0110]FIG. 71 is a graph for the relationship between the spectrum ofFIG. 69 and the spectrum of FIG. 70 in the fourth embodiment;

[0111]FIG. 72 is a graph for the spectrum of the hologram when R/O is 5,and when the peak wavelength is 460 nm, in the fourth embodiment;

[0112]FIG. 73 is a graph for the relationship between the spectrum ofFIG. 69 and the spectrum of FIG. 72 in the fourth embodiment;

[0113]FIG. 74 is a graph for the relationship between the amount ofangle correction and the peak efficiency in the fifth embodiment;

[0114]FIG. 75 is an explanatory view for the essential parts of theexposure optical system in the six embodiment;

[0115]FIG. 76 is an explanatory view for a measuring method of thescattering extent of the object light in the sixth embodiment;

[0116]FIG. 77 is a graph for the scattering extent of the object lightin the sixth embodiment;

[0117]FIG. 78 is a graph for the relationship between a sum of theexposure intensity and the half bandwidth in the seventh embodiment;

[0118]FIG. 79 is a graph for the relationship between the sum of theexposure intensity and the peak wavelength in the seventh embodiment;

[0119]FIG. 80 is a graph for the relationship between the heatingtemperature and the peak wavelength in the eighth embodiment;

[0120]FIG. 81 is a graph for the relationship between the heatingtemperature and the half bandwidth in the eighth embodiment;

[0121]FIG. 82 is a graph for the relationship between the heating timeand the half bandwidth in the eighth embodiment;

[0122]FIG. 83 is an explanatory view for a reducing method of thethickness of the hologram element by using a laminator;

[0123]FIG. 84 is a graph for the relationship between the efficiency andthe wavelength in the spectrum of the hologram;

[0124]FIG. 85 is an explanatory view for the relationship between ashowroom and the hologram screen according to a first embodiment of thefourth aspect of the present invention;

[0125]FIG. 86 is a graph for the relationship between a haze ratio andthe number of observers, i.e., evaluation level when reading characterson the screen by observers in the first embodiment;

[0126]FIG. 87 is a graph for the relationship between the haze ratio andthe number of observers, i.e., evaluation level of cloudiness on thescreen in the first embodiment;

[0127]FIG. 88 is an explanatory view for the exposure optical system inthe second embodiment of the fourth invention;

[0128]FIG. 89 is a graph for the relationship between the output angleof the diffusion body and the intensity ratio at various scatteringangles in the second embodiment;

[0129]FIG. 90 is a graph for the relationship between the intensityratio E_(R)/E_(O) and the haze ratio at various scattering angles of thelight diffusion body in the second embodiment;

[0130]FIG. 91 is a graph for the relationship between the scatteringangle of the light diffusion body and the intensity ratio E_(R)/E_(O) atthe different haze ratio in the second embodiment;

[0131]FIG. 92 is a graph for the relationship between the intensityratio E_(R)/E_(O) and the haze ratio at various scattering angles of thelight diffusion body in the second embodiment;

[0132]FIG. 93 is a graph for the relationship between the intensityratio E_(R)/E_(O) and the haze ratio when using the light diffusion bodyhaving the scattering angle of 36° in the second embodiment;

[0133]FIG. 94 is a graph for the relationship between the scatteringangle of the light diffusion body and the intensity ratio E_(R)/E_(O)when producing the hologram screen having the haze ratio of 5% in thesecond embodiment;

[0134]FIG. 95 is a graph for the relationship between the intensityratio E_(R)/E_(O) and a diffraction efficiency η_(RO) of the normalinterference fringe at the hologram screen which is produced by usingthe light diffusion body having the scattering angle of 36° in thesecond embodiment;

[0135]FIG. 96 is an explanatory view for a measuring method of thediffraction efficiency in the second embodiment;

[0136]FIG. 97 is a graph for the reproduced wavelength and thetransmission ratio in order to calculate the diffraction efficiency;

[0137]FIG. 98 is a graph for the relationship between the scatteringangle of the light diffusion body and the intensity ratio E_(R)/E_(O) inthe second embodiment;

[0138]FIG. 99 is a graph for the relationship between the scatteringangle of the light diffusion body and the screen gain in the secondembodiment;

[0139]FIG. 100 is a graph for the relationship between the amount ofexposure and the diffraction efficiency η_(RO) of the interferencefringe in the second embodiment;

[0140]FIG. 101 is a graph for the relationship between the amount ofexposure and the haze ratio of the hologram screen in the secondembodiment;

[0141]FIG. 102 is a graph for the relationship between the thickness ofthe photosensitive material and the haze ratio of the hologram screen inthe second embodiment;

[0142]FIG. 103 is an explanatory view for the beam of light toward theobserver from the object existing at the back of the light diffusionbody;

[0143]FIG. 104 is an explanatory view for the beam of light toward theobserver from the object existing at the back of the hologram screen;

[0144]FIG. 105 is an explanatory view for the diffraction of externallight due to the interference fringe on the hologram screen;

[0145]FIG. 106 is an explanatory view when forming the interferencefringe on the photosensitive material;

[0146]FIG. 107 is an explanatory view when forming the “Fresnel noise”on the photosensitive material;

[0147]FIG. 108 is a graph for the relationship between the output angleand the intensity of output light at the normal light diffusion body andthe hologram screen;

[0148]FIG. 109 is an explanatory view in the case when a plurality ofobject lights and one reference light are irradiated to the sameposition on the photosensitive material;

[0149]FIG. 110 is an explanatory view for the display apparatus usingthe transmission type hologram screen;

[0150]FIG. 111 is an explanatory view for the display apparatus usingthe reflection type hologram screen;

[0151]FIG. 112 is an explanatory view for indicating a basic structureof the transmission type hologram screen; and

[0152]FIG. 113 is an explanatory view for indicating a basic structureof the reflection type hologram screen.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0153] Before explaining preferred embodiments of the present invention,the conventional arts and their problems will be explained in detailwith reference to the attached drawings.

[0154]FIG. 1 is an explanatory view of the use of a hologram screen. Inthe drawing, an observer 18 watches a hologram screen 11, and an imageprojector 112 is arranged to an upper location at the back of thehologram screen 11. An image light 1120 is irradiated from the imageprojector 112 to the hologram screen 11. The image light 1120 isscattered and diffused by the hologram screen 11 to the observer's sideso as to obtain an output light 1121. The image can be reproduced by theoutput light 1121.

[0155] On the other hand, it is possible to constitute the hologramscreen 11 by using a transparent material. In the case of thetransparent hologram screen, the observer can observe not only the imagereproduced on the hologram screen 11, but also an object 119 located atthe back of the hologram screen 11. As mentioned above, the hologramscreen 11 can be widely utilized as a display apparatus having very goodvisual effect.

[0156] The hologram screen 11 can be produced by using an exposureoptical system 12 shown in FIG. 10 which is explained in detail below.Briefly, a laser oscillator 1210 generates a laser beam, and the laserbeam is splitted by a beam splitter 1211. One light 1212 is irradiatedto a light diffusing body 1216 through an object tens 1222 and aparabolic mirror 1214. The diffusion light from the light diffusing body1216 is used as an object light 1217. On the other hand, the other light1213 from the beam splitter 1211 is used as a reference light 1218through an object lens 1221.

[0157] The object light 1217 and the reference light 1218 are irradiatedon a photosensitive material 1220 so that an interference fringe formedby these lights 1217 and 1218 is recorded on the photosensitive material1220. As a result, the light diffusing body 1216 is recorded on thephotosensitive material 1220. When the image light 1120 (see FIG. 1) isirradiated, the light diffusing body 1216 is reproduced. Then, thediffusing light is output from the light diffusing body 1216, and at thesame time, the image light 1120 is diffracted and scattered on thehologram screen 11 so that it is possible to reproduce the image on thehologram screen 11.

[0158] In the exposure optical system 2 in FIG. 10, an incident angle θ₀of the reference light 1218 to a center 1229 of the photosensitivematerial 1220 and an incident distance R₀₀ of the reference material1220 are approximately equal to an incident angle f₀ of the image light1120 to the hologram screen 11 and an incident distance F₀ of the imagelight 1120 in FIG. 1.

[0159] However, when the image is reproduced by the conventionalhologram screen, for example, although approximately the same color toneas the image light can be obtained at the center of the hologram screen,the color tone becomes more blue than the image light at the upperportion of the hologram screen, and becomes more yellow than the imagelight at the lower portion of the hologram screen (see COMPARING SAMPLEC1 in the third embodiment). That is, in the conventional art, it isimpossible to reproduce uniformly the color of the image light on allareas of the hologram screen. That is, there is a problem in which anunnatural image which gives large feeling of difference to the observeris reproduced in the conventional art.

[0160] Accordingly, the first invention aims to provide a hologramscreen which can eliminate unevenness of color on the reproduced image,and a producing method of the hologram screen as explained in detailbelow.

[0161] Explanations of the First Aspect of the Present Invention

[0162] According to the invention defined in claim 1, in the hologramscreen for reproducing the image based on the output light by scatteringand diffusing the image light which is irradiated from the imageprojector, when a white light is projected on the hologram screen as theimage light, and when a value on the CIE 1976 chromaticity coordinate(u′, v′) is (u′_(A), v′A) and (u′_(B), v′_(B)) at optional two points Aand B having the distance of 20 cm or less between two points on thehologram screen,

[0163] the hologram screen is characterized in that the output lightwhich is output perpendicularly from the surface of the hologram screenhas color distribution in which the color difference Δu′v′ between twopoints A and B and delivered from the following formula (1) becomes 0.06or less.

[0164] where, the formula (1) is expressed by,

Δu′v′ =[(u′ _(A) −u′ _(B))²+(v′ _(A) −v′ _(B)))]^(1/2)  (1)

[0165] As mentioned above, A and B are optional two points on thesurface of the hologram (i.e., the side in which the output light isoutput), and the distance between two points is 20 cm or less (see FIG.2). The color difference at two points can be obtained from thechromaticity coordinate at two points. When the color difference islarger than 0.6, unevenness of color may occur on the reproduced image.Further, it is preferable that the color difference is nearly zero. Thatis, when the color difference is brought near to zero, it is possible toalmost eliminate unevenness of color of the image. In this case, the CIE1976 chromaticity coordinate (u′, v′) indicates a value on an orthogonalcoordinate of u′ and v′ which is obtained from the UCS chromaticitycoordinate defined by the CIE.

[0166] The operation of the first aspect of the present invention willbe explained below.

[0167] The important point in the first aspect of the present inventionlies in that the output light has the color distribution in which thecolor difference Δu′v′ between optional two points A and B having thedistance of 20 cm or less becomes 0.06 or less. The maximum colordifference of the output light is 0.06 in the present invention, and thedifference of the color between the output lights irradiated from eachportion is small. Accordingly, the output light from any position on thehologram screen has an approximately uniform color tone so that it ispossible to obtain the output light in which the color of the imagelight is considered. As a result, it is possible to prevent unevennessof color on the hologram screen, such as a blue portion at the upper onthe screen and a yellow portion at the lower on the screen, whenreproducing the image.

[0168] The hologram screen of the first aspect of the present inventioncan reproduce the color of the image light irradiated on the screen, andit is possible to reproduce the image having no feeling of differencefor the observer. Accordingly, the hologram screen according to thefirst aspect of the present invention can be widely utilized as thescreen of various image display apparatus. Further, since it is easy toconstitute the hologram screen by using the transparent material, thepresent invention can be utilized as a display apparatus for displayingboth image on the hologram screen and image transmitted from the back.Further, when the image is not displayed on the hologram screen, it ispossible to utilize a screen in which the view field is not cut off(i.e., the view field can see through the hologram screen because of thetransparent material).

[0169] The hologram screen according to the first invention can beutilized as a screen for advertisement provided for various show windowsor a screen for navigation system provided in a vehicle, etc. Further,as the image projecting apparatus 112, it is possible to utilize variousprojectors and liquid crystal projectors. Still further, as the imagelight, it is possible to utilize various advertisements andenvironmental images without kinds of images. Still further, regardingthe color difference, it is preferable to satisfy the color differencenot only in the case of the front of the hologram screen (see FIG. 3),but also in the cases of upper, lower, left and right directions of thehologram screen.

[0170] According to the invention defined in claim 2, the hologramscreen is characterized in that the output light output from the surfaceof the hologram screen to a viewpoint provided based on the followingformula (2) has color distribution in which the color difference Δu′v′between two points A and B and delivered from the above formula (1)becomes 0.1 or less.

[0171] where, the formula (2) is expressed by,

H/2L=0.1  (2)

[0172] In this case, L is the distance between the viewpoint and thecenter of the hologram screen, and H is a height of the hologram screenitself.

[0173] When the color difference is larger than 0.1, unevenness of colormay occur on the reproduced image. Further, it is preferable that thecolor difference is nearly zero. That is, when the color difference isbrought near to zero, it is possible to almost eliminate unevenness ofcolor of the image. The maximum color difference of the output lighttoward the viewpoint determined based on the formula (2) is 0.1, and thedifference of the color between each output lights is small.Accordingly, when the image light has a single color, the output lightgenerated from any portion on the hologram screen has approximately thesame color tone each other. As a result, it is possible to obtain theoutput light in which the color of the image light is reflected. Asmentioned above, as a result, it is possible to prevent unevenness ofcolor on the hologram screen, such as a blue portion at the upper on thescreen and yellow portion at the lower on the screen, when reproducingthe image.

[0174] According to the invention defined in claim 3, it is preferablethat, as the spectral characteristic of the output light which is outputperpendicularly from the surface of the hologram screen, either thedifference in peak wavelengths at all areas of the hologram screen is120 nm or less, or the difference in half bandwidth is 100 nm or less.

[0175] When the difference in the peak wavelength is larger than 120 nm,the output light from the hologram screen is constituted by the lighthaving more wide range of the wavelength. As a result, the difference ofthe color on the hologram screen become large so that unevenness ofcolor easily occurs in the reproduced image. Further, it is preferablethat the difference in the peak wavelength is nearly zero.

[0176] When the half bandwidth is larger than 100 nm, the wavelengthdistribution of the output light from the hologram screen becomesnarrow, and the wavelength distribution from another portion becomeswide. That is, the output light from the hologram screen is constitutedby the light having more wide range of the wavelength so that thedifference in the color becomes large and unevenness of color easilyoccurs in the reproduced image. Further, it is preferable that thedifference in the half bandwidth is nearly zero.

[0177] In the above explanation, the description “the difference in peakwavelengths at all areas of the hologram screen is 120 nm or less” meansthat, when collecting the spectral characteristics of the output lightfrom the hologram screen, the difference in the peak wavelength betweenone spectral characteristic having longest peak wavelength and anotherspectral characteristic having shortest peak wavelength is 120 nm.

[0178] Regarding the half bandwidth, the description “the difference inhalf bandwidth is 100 nm or less” means that the difference in halfbandwidth between one spectral characteristic having the most wide halfband and another spectral characteristic having the most narrow halfband is 100 nm.

[0179] According to the invention defined in claim 4, in the exposureoptical system using the object light and the reference light, theobject light being the diffusion light obtained by using the lightdiffusion body and the reference light being a non-diffusion light, thehologram screen defined in claim 1 or 2 is produced by irradiating theobject light and the reference light onto the photosensitive material inorder to expose the photosensitive material.

[0180] At that time, a method of producing the hologram screen accordingto the present invention is characterized in that the photosensitivematerial having a thickness distribution which is inclined to theincident direction of the reference light (see 1220 in FIG. 11). Thismeans that the thickness of the photosensitive material is differentbetween the upper portion and the lower portion. In this case, theincident direction of the reference light is indicated by the directions(1), (2) and (3) in FIG. 11.

[0181] In this case, there is a known fact that the half bandwidthbecomes wider in the spectral characteristic of the hologram screen madeby the thin photosensitive material, and the spectral characteristicbecomes broader. This is obvious from the following logical formula fordefining the half bandwidth.

Δλ=λ₀ ² /[T(n−(n ²−sin² θ_(r))^(1/2))]  (3)

[0182] where, Δλ is a half bandwidth, λ₀ is a recorded wavelength, T isa thickness of the photosensitive material, n is a refractive index ofthe photosensitive material, and θ_(r) is an incident angle of thereference light.

[0183] The spectral characteristic of the hologram screen made by thephotosensitive material having the uniform thickness in a conventionalart is shown in FIG. 14. In this spectral characteristic, the halfbandwidth becomes broader in order of (1)→(2)→(3). For example, whenputting all spectral characteristics into the spectral characteristic(2), the photosensitive material for the spectral characteristic (1)having a narrow half bandwidth is thinned, and the photosensitivematerial for the spectral characteristic (3) having a broad halfbandwidth is thickened.

[0184] As mentioned above, it is possible to obtain the hologram screenhaving the uniform spectral characteristic by providing thephotosensitive material having the different thickness between the upperportion and the lower portion.

[0185] In this case, the difference in the color of the output lightfrom each portion of the hologram becomes small so that it is possibleto obtain the output light having almost the same color tone.

[0186] As is obvious from the above, according to the invention definedin claim 4, as well as the invention defined in claims 1 and 2, it ispossible to provide a producing method of the hologram screen having nounevenness of color in the reproduced image. Further, the presentinvention has the same effect as the above, for not only the incidentdirection of the reference light, but also an orthogonal direction,i.e., the right and left directions when reproducing the image on thehologram screen.

[0187] As the photosensitive material, it is possible to utilize variousmaterials, for example, various kinds of photopolymers, a dichromatedgelatin, a silver salt, etc. Further, as the light diffusion body, it ispossible to utilize any kinds of materials, for example, a ground glass,an opal glass, etc.

[0188] Next, according to the invention defined in claim 5, as thephotosensitive material, it is preferable to use the material in whichthe larger the incident angle of the reference light, the larger thethickness distribution.

[0189] As shown in FIG. 11, the incident angle θ of the reference light1218 in the exposure optical system becomes small at the location nearto a divergence point 1221 of the reference light, and becomes large atthe location apart from the divergence point 1221. That is, the incidentangle θ₁ is the smallest, and the incident angle θ₃ is the largest, asshown in the drawing. Accordingly, in the method defined in claim 5, thephotosensitive material having the thickness which becomes graduallythick from the location near to the divergence point to the locationapart from the divergence point, is used as the hologram screen.

[0190] When the half bandwidth of the spectrum characteristic of theoutput light from the hologram screen is widened, the wavelengthdistribution of the output light can be broadened so that it is possibleto increase reappearance of the color of the image light. Further, it ispossible to avoid reduction of the reappearance of the color of theimage light caused by the difference of the peak wavelength in thespectrum characteristic.

[0191] As mentioned above, when the thickness of the photosensitivematerial becomes thin, the spectral characteristic of the output lightbecomes broad. That is, selectivity of the wavelength for the incidentlight becomes small so that it is possible to diffract the broaderwavelength (i.e., the half bandwidth becomes broad.). However, since thenumber of the interference fringe which can record is reduced in thecase of the thin photosensitive material, the hologram screen having loweffectivity is obtained.

[0192] On the other hand, when the thickness of the photosensitivematerial becomes thick, the selectivity of the wavelength becomes large(i.e., the half bandwidth becomes narrow.). However, since the number ofthe interference fringe which can record is increased in thephotosensitive material, the hologram screen having high effectivity canbe obtained. Accordingly, by providing the thickness distribution havingdifferent thickness at the upper portion and the lower portion, it ispossible to arrange all spectrum distributions of the hologram screen,and to match the whole efficiency of the hologram screen. In this case,the difference of the color of the output light from each portion of thehologram screen becomes small so that it is possible to obtain theoutput light having almost the same color tone.

[0193] According to the invention defined claim 6, it is preferable thatthe thickness distribution of the photosensitive material is given byΔT≦0.5 T₀, where, ΔT is a difference in thickness between the upper endand the lower end in the photosensitive material. According to thisconditions, it is possible to ensure the effect of the presentinvention. If the ΔT is larger than 0.5 T₀, the range of the thicknessdistribution of the hologram screen becomes too wide so that it may bevery difficult to obtain the effect of reduction of unevenness of color.

[0194] It is preferable that an inclined degree k of the thickness ofthe photosensitive material is given by 0<k≦2×10⁻⁵. Where, the inclineddegree k is given by a value when a primary approximation is executed tothe thickness distribution of the photosensitive material. When theinclined degree “k” of the thickness becomes larger than 10^(−5,) anamount of shift of the peak wavelength in the spectrum distribution ofthe output light becomes too large so that the difference occurs in thecolor of the output light and the improved effect of unevenness of colormay be reduced.

[0195] According to the invention defined in claim 7, it is preferableto establish the following formula (4) in the thickness distribution,i.e.,

T=a(R ₀ −R)+T ₀ , a=b·θ ₀ ^(−0.9)  (4)

[0196] where, T is the thickness at optional location of thephotosensitive material, T₀ is the thickness at a center of thephotosensitive material, R is an incident distance of the referencelight at optional location of the photosensitive material, R₀ is theincident distance of the reference light at the center of thephotosensitive material, θ₀ is the incident angle of the reference lightat the center of the photosensitive material, and “b” is a coefficientdetermined by the thickness distribution and defined as 0<b<1.

[0197] The photosensitive material having the thickness distributionsatisfying the above conditions becomes linearly thin from the thickportion until the thin portion. By using the above photosensitivematerial, the brightness and color on the image are not changedpartially, and it is possible to obtain the hologram screen in which,for example, stripe-like unevenness of color does not occur on thescreen.

[0198] In the thickness distribution, even if there is unevenness in therange of 15% of the thickness T₀ at the center of the photosensitivematerial, there is no problem of the stripe-like unevenness of color.

[0199] According to the invention defined in claim 8, in the exposureoptical system using the object light and the reference light, theobject light being the diffusion light obtained by using the lightdiffusion body and the reference light being the non-diffusion light,the hologram screen defined in claim 1 or 2 is produced by irradiatingthe object light and the reference light onto the photosensitivematerial in order to expose the photosensitive material.

[0200] At that time, a method of preparing the hologram screen accordingto the present invention is characterized in that the photosensitivematerial which is previously irradiated by an ultraviolet in accordancewith energy distribution inclined to the incident direction of thereference light is used as the hologram screen.

[0201] Further, there is a material which can stop polymerization actionof monomer by irradiating the ultraviolet in the photosensitivematerial. For this material, it is possible to reduce the sensitivity bypreviously irradiating the ultraviolet having the intensity in which thepolymerization action of the monomer is not completely stopped.

[0202] According to the invention defined in claim 9, it is preferableto satisfy the following formula (5) as the energy distribution of theultraviolet regarding the incident distance R of the reference light atthe optional location on the photosensitive material.

0.8E≦Euv≦1.2E, and E=0.01−(R−R ₀)+E ₀  (5)

[0203] where , Euv is an amount of energy of the ultraviolet irradiatedto the optional location on the photosensitive material, R₀ is anincident distance of the reference light at the center of thephotosensitive material, and E₀ is an amount of energy of theultraviolet irradiated to the center of the photosensitive material. Asa result, it is possible to obtain the photosensitive material havingthe optimum sensitivity.

[0204] According to the invention defined in claim 10, in the exposureoptical system using the object light and the reference light, theobject light being the diffusion light obtained by using the lightdiffusion body and the reference light being the non-diffusion light,the hologram screen defined in claim 1 or 2 is produced by irradiatingthe object light and the reference light onto the photosensitivematerial in order to expose the photosensitive material.

[0205] At that time, a method of producing the hologram screen accordingto the present invention is characterized in that the incident distanceof the reference light at the optional location of the photosensitivematerial is shorter than the incident distance of the image light inputfrom the image projector when reproducing the image on the hologramscreen.

[0206] The incident angle θ₀ and the incident distance R₀ at the centerof the photosensitive material when producing the hologram screen isusually set to the same value as an incident angle f₀ and an incidentdistance F₀ at the center of the hologram screen when irradiating theimage light on the hologram screen and reproducing the image (see FIGS.1 and 10).

[0207] In the producing method according to the present invention, theincident distance Ro of the reference light is set to the distanceshorter than the incident distance F₀ so that it is possible to set theincident angle of the reference light, except for the center of thephotosensitive material, to a value different from the incident angle ofthe image light.

[0208] That is, the incident angle of the image light coincides with theincident angle of the reference light at the center of thephotosensitive material, but, except for the center, the incident angleof the image is different from the incident angle of the referencelight. At that time, the difference in distance of the above is changedin accordance with the distance from the center of the photosensitivematerial.

[0209] As shown in FIG. 20, the incident distance R₀ is shortened to theincident distance R₀′at the center 1229 of the photosensitive material1220. As a result, the incident angle θ_(R1), at the end (1) of thephotosensitive material 1220 becomes small, and the incident angleθ_(R3) at the end (3) (a side opposite to the end (1)) of thephotosensitive material 1220 becomes large.

[0210] When the incident angle of the reference light is different fromthe incident angle of the image light, there is known wavelength shiftwhich is expressed by the following logical formula (6) in the spectrumcharacteristics of the hologram screen.

sin θ_(z)=sin θ_(y)+μ(sin θ_(x)−sin θ₀), and

μ=λ_(z)/λ₀  (6)

[0211] As shown in FIGS. 20 and 21, θ_(R1) is the incident angle at theend (1) of the reference light, θ_(x) is an incident angle of the objectlight input from the point X1 of the light diffusion body 1216 to thepoint X2 of the photosensitive material 1220, λ₀ is a wavelength of thelaser used in the exposure. Further, as shown in FIG. 22, λ_(y) is anincident angle of the image light input to the point X2 of the hologramscreen 11, θ_(z) is an angle of the output light 1121 diffracted by thehologram screen and output toward the direction of the arrow, and λ_(z)is wavelength of the output light 1121.

[0212] From the above formula (6), the following formula can beobtained.

λ_(z)=λ₀[(sin θ_(z)−sin θ_(y))/(sin θ_(x)−sin θ_(R1))

[0213] As is obvious from the above formula, when the incident angleθ_(R1) of the reference light is smaller than the incident angle θ_(y)of the image light, the output light is shifted to a side of the longwavelength. On the other hand, when the incident angle θ_(R1) is largerthan the incident angle θ_(y), the output light is shifted to the sideof the short wavelength.

[0214] As the hologram screen in which the incident angle of thereference light is equal to the incident angle of the image light, thereis a comparing sample C1 explained in the third embodiment. The spectralcharacteristic of this sample is shown in FIG. 14. If the spectralcharacteristic of this hologram screen is arranged to the spectralcharacteristic of the curve (2), the spectral characteristic of thecurve (1) is shifted to the side of the long wavelength and the spectralcharacteristic of the curve (3) is shifted to the side of the shortwavelength.

[0215] That is, the incident angle of the reference angle is set to theangle smaller than the incident angle of the image light in the spectralcharacteristic (1), and the incident angle of the reference angle is setto the angle larger than the incident angle of the image light in thespectral characteristic (3). This can be realized by shortening theincident distance of the reference light than the incident distance ofthe image light. In this case, the incident angle of the reference lightis maintained to the same value of the incident angle of the image lightat the center of the photosensitive material.

[0216] As a result, since the spectral characteristic of the hologramscreen can be arranged in the whole, the difference of color of theoutput light from each portion of the hologram screen becomes small, andit is possible to obtain the output light having approximately the samecolor tone.

[0217] According to the invention defined in claim 11, in the exposureoptical system using the object light and the reference light, theobject light being the diffusion light obtained by using the lightdiffusion body and the reference light being the non-diffusion light,the hologram screen defined in claim 1 or 2 is produced by irradiatingthe object light and the reference light onto the photosensitivematerial in order to expose the photosensitive material.

[0218] At that time, a method of producing the hologram screen accordingto the present invention is characterized in that an intensity ratioI_(R)/I_(O) of the reference light and the object light has thefollowing relationship is given by the following formula, i.e.,I_(R)/I_(O)≦10, where, I_(R) is the intensity of the reference light,and I_(O) is the intensity of the object light.

[0219] In this embodiment, the ratio I_(R)/I_(O) is 10 or less. When theintensity ratio I_(R)/I_(O) becomes large, the intensity of the objectlight becomes very weak. As a result, the dispersion of the spectralcharacteristic, which is caused by the dispersion of the intensity ofthe object light due to the location on the photosensitive material, iseasy to occur on the hologram screen. That is, the difference of thespectral characteristic due to the intensity of the object light becomeslarge.

[0220] When the intensity of the object light becomes large, thespectral characteristic becomes approximately constant so that thesmaller the ratio I_(R)/I_(O) the smaller the difference of the spectralcharacteristic. As a result, it is possible to arrange the spectralcharacteristic in the whole of the hologram screen. Further, thedifference of color of the-output light from each portion of thehologram screen becomes small so that it is possible to obtain theoutput light having approximately the same color tone.

[0221] According to the invention defined in claim 12, in the exposureoptical system using the object light and the reference light, theobject light being the diffusion light obtained by using the lightdiffusion body and the reference light being the non-diffusion light,the hologram screen defined in claim 1 or 2 is produced by irradiatingthe object light and the reference light onto the photosensitivematerial in order to expose the photosensitive material.

[0222] At that time, a method of producing the hologram screen accordingto the present invention is characterized by at least two sets of thefollowing features (A) to (D), i.e.,

[0223] (A): to use the photosensitive material having the thicknessdistribution inclined to the incident direction of the reference light,

[0224] (B): to use the photosensitive material which is previouslyirradiated by the ultraviolet in accordance with the energy distributioninclined to the incident direction of the reference light,

[0225] (C): to determine the incident distance of the reference light atthe optional location on the photosensitive material so as to becomeshorter than the incident distance of the image light input from theimage projector when reproducing the image on the hologram screen, and

[0226] (D): to have the relationship of the ratio I_(R)/I_(O)<10 in theintensity of the reference light and the object light in the whole ofthe surface of the photosensitive material.

[0227] According to the features (A) to (D), it is possible to obtainthe hologram screen having the arranged spectral characteristic. Asmentioned above, by performing at least two sets of features (A) to (D),it is possible to make the difference of color eye small, and to satisfythe desired hologram characteristic, although it is very difficult torealize small difference of color in the case of any one feature from(A) to (D) depending on the desired hologram characteristic (forexample, a view angle, a brightness of an image to be reproduced, etc.).

[0228] Explanations of Preferred Embodiments of the First Aspect of thePresent Invention

[0229] (First Embodiment)

[0230] The first embodiment will be explained in detail with referenceto FIGS. 1 to 7.

[0231] As shown in FIG. 1, the hologram screen 11 reproduces the imagebased on the output light 1121 obtained by scattering and diffusing theimage light 1120 irradiated from the image projector 112. As explainedabove, when the white light is projected on the hologram screen 11 asthe image light, and when a value on the CIE 1976 chromaticitycoordinate (u′, v′) is (u′_(A), v′_(A)) and (u′_(B), v′_(B)) at optionaltwo points A and B having the distance D_(AB) of 20 cm or less betweentwo points A and B on the hologram screen, as shown in FIG. 2,

[0232] the output light 1121 output perpendicularly from the surface ofthe hologram screen has color distribution in which the color differenceΔu′v′ between two points A and B and delivered from the above formula(1) becomes 0.06 or less.

[0233] Next, one example in use of the hologram screen will be explainedbelow. As shown in FIG. 1, the image projector 112 is provided at theupper back of the hologram screen 11 for the observer 18 who observesthe hologram screen 11. The image light 1120 is irradiated from theimage projector 112 to the hologram screen 11. As a result, the imagelight 1120 is scattered and diffused by the hologram screen 11 to theside of the observer 18 so that the output light 1121 is obtained andthe image can be reproduced by the output light 1121. Further, theincident distance of the image light is expressed by F₀, and theincident angle of the image light is expressed by f₀.

[0234] Further, it is possible to prepare the hologram screen 11 byusing the transparent material. In this case, the observer 18 canobserve not only the image reproduced by the hologram screen 11, butalso the object 119 at the back of the hologram screen 11. The hologramscreen 11 in FIG. 1 is a transmission type, but it is possible to use areflection type. When the reflection type is used, the image projector112 is provided to the same side as the observer 18.

[0235] The hologram screen according to the first embodiment was testedin accordance with the following processes.

[0236] First, the hologram screen 11 having a size of 300 nm×400 nm wasprepared. In this case, as shown in FIG. 2, in optional two square areasA and B (both 30 mm×30 mm, each point A and B denotes a center of thesquare area) on the hologram screen 11, and the maximum color differenceΔu′v′ between the points A and B is 0.01 to 0.08.

[0237] The color difference Δu′v′ was measured in accordance with thefollowing steps.

[0238] As shown in FIG. 3, the white light 1130 is irradiated to thehologram screen 11. In the output light diffracted by each point (1),(2) and (3) on the hologram screen 11, in particular, the spectrumcharacteristic of the output light, which is irradiated perpendicularlyto the surface of of the hologram screen 11 at the output side, wasmeasured by a light receiving device 1179. In this case, a point 1139 isthe location of the projector of the white light 1130.

[0239] Further, as shown in FIG. 1, the image is reproduced byirradiating the image light 1120 onto the hologram screen 11. In thiscase, the incident angle of the reference light to the hologram screen11 is equal to the incident angle of the white light 1130 to thehologram screen 11 shown in FIG. 3.

[0240] Next, how to obtain the color difference Δu′v′ using the spectralcharacteristic is explained below. In this case, the CIE 1976chromaticity coordinate (u′, v′) is used in the following steps.

[0241] The chromaticity coordinate u is expressed below, i.e.,

u′=4X/(X+15Y+3Z)  (7)

[0242] In the above formula, for example, X can be obtained from theformula shown in FIG. 4. Y and Z can be obtained similarly from theformula shown in FIG. 4. In this case, S(λ) was used as the spectralcharacteristic of the light after the white light of a liquid crystalprojector has passed through the liquid crystal. This is because it ispossible to perform evaluation of color of the image displayed on thehologram screen 11, as well as evaluation of unevenness of color, whenthe S(λ) is used as the spectral characteristic of the light having thesame light source as that of the image projector actually used. Further,T(λ) denotes a spectral transmission rate and it was used as thespectrum characteristic of the hologram screen obtained by measurementin FIG. 3.

[0243] As a result of the above measurement, next test was performedusing the hologram screen having the maximum color difference of 0.01 to0.08. In this case, although the color difference can be obtained fromthe spectral characteristic, it is possible to obtain the colordifference by directly measuring the chromaticity using a calorimeter.

[0244] As shown in FIG. 5, the liquid projector was used as the imageprojector 112, and the white light was used as the image light 1120.Further, many observers, for example, twenty observers, observed theimage during ten minutes at the location two meters apart from thecenter 1109 of the hologram screen 11.

[0245] A result of observation is shown in FIG. 6. The image wasevaluated by twenty observers in accordance with a subjective evaluationgrade having seven grades 1 to 7 (see 1 to 7 in FIG. 6). In this case,ten minutes is required for getting good condition for eye and correctlyrecognizing presence or absence of unevenness of color. The measurementwas performed within a room, and no lamp was provided in the room. Thebrightness of the white color to be projected was 300 cd/m². A frostedblack wall 119 was provided at the back of the hologram screen 11.

[0246] As shown in FIG. 6, for the hologram screen having the maximumcolor difference of 0.05 or less, all observers do not clearly recognizeunevenness of color (see levels 5 to 7). When the color difference is0.06, some observers can recognize unevenness of color, but theyevaluate this unevenness of color as an ambiguous feeling, i.e., “domind” or “don't mind” (see level 4).

[0247] In this test, no lamp is provided and the background light is theblack which is very close to zero. When the illuminance within the roomis 500 Lux (this is an average illuminance in offices), and when thewhite wall paper is used at the back of the hologram, the level 4 inthis test becomes the same level as the level 5 or more, i.e., theobserver does not mind unevenness of color. Further, when the colordifference is 0.07 or more, there is the observer who does mindunevenness of color in this test. As is obvious from the above, when thecolor difference is 0.06 or less, it is obvious that the observer doesnot mind unevenness of color of the image on the hologram screen.

[0248] Next, the effect of the first embodiment will be explained below.As mentioned above, the output light has color distribution in which thecolor difference Δu′v′ between points A and B is 0.06 or less. That is,since the difference of color is small, the output light from anylocation on the hologram screen has the same color tone each other, ifthe incident light is the monochrome. That is, there is no phenomenonfrom which color at a part of the hologram screen are different. As aresult, as shown in FIG. 6, there is no occurrence of unevenness ofcolor on the hologram screen.

[0249] The spectral characteristic was checked for the hologram screenhaving the color difference 0.06. In this case, when the half bandwidthis the same, the difference of the peak wavelength corresponds toapproximately 120 nm. On the other hand, when the peak wavelength is thesame, the difference of the half bandwidth corresponds to approximately100 nm. As is obvious from the above when the color difference on thehologram screen is not measured, the conditions for the color differencecan be satisfied if the spectral characteristic is smaller than theabove difference.

[0250] As shown in FIG. 7, it is possible to measure the spectralcharacteristic of the output light emitted toward the direction G whenthe light receiving device 1179 is provided at the angle “g” for theperpendicular direction from the surface of the hologram screen 11. Evenif the hologram screen has color eye distribution in which the colordifference Δu′v′ obtained from the above spectral characteristic becomes0.06 or less, there is no unevenness of color on the reproduced image.

[0251] (Second Embodiment)

[0252] In the hologram screen according to the second embodiment, theoutput light, which is emitted toward the viewpoint, has the colordistribution in which the color difference between two points is 0.1 orless. As well as the first embodiment, shown in FIG. 1, the image isreproduced based on the output light by scattering and diffusing theimage light from the image projector. As mentioned above, the outputlight emitted toward the viewpoint based on the formula (2) has thecolor eye distribution in which the distance between two points A and Bis 20 cm or less, and the color difference Δu′v′ is 0.1 or less.

[0253] As shown in FIG. 8, when the distance between the viewpoint 180and the center 1109 of the hologram screen 11 is given by L, and whenthe height of the hologram screen 11 is given by H, the formula (2),i.e., H/2L=0.1, is given as mentioned above.

[0254] The measurement of the color difference will be explained indetail below. As shown in FIG. 8, the white light 1130 is irradiated onthe hologram screen 11. When the white light 1130 is input by theincident angle θ_(c) at the center 1109 on the hologram screen 11, thewhite light 1130 is input by the incident angle θ_(c) at the point 1108,i.e., at a point except for the center 1109.

[0255] The white light 1130 is diffracted on the hologram screen 11 sothat the output light 1131 can be obtained. In this case, the spectralcharacteristic of the output light 1131 at the location apart by thedistance L from the center 1109 of the hologram screen 11 was measured.In order to measure the spectral characteristic of the output light1131, first, the output light from the point 1108 is measured byproviding the light receiving device 1179 on the line between theviewpoint 180 and the point 1108. That is, the light receiving device1179 is inclined by the same angle as that of the line between theviewpoint 180 and the point 1108. As a result, the spectralcharacteristic is measured, and the color difference can be obtained bythe same method as the first embodiment. In this case, the output lightfrom the center 1109 is output perpendicularly from the surface of thehologram screen 11, as well as the first embodiment.

[0256] The color difference was re-checked based on the above methodregarding the hologram screen having the color difference 0.06 in thefirst embodiment. As a result, when H=300 mm, and L=2000 mm, in thestructure of the hologram screen, the color difference was changed from0.06 to 0.1 in the second embodiment. Further, since the hologram screenhaving the color difference 0.06 in the first embodiment was evaluatedas the level 5 or more (i.e., the observer does not mind unevenness ofcolor), it is obvious that there is no unevenness of color on thereproduced image for the hologram screen having the color difference0.1. Since these measuring methods can be used to measure the colordifference which can be observed actually through a human eye, it ispossible to perform practical evaluation.

[0257] As shown in FIG. 9, the spectral characteristic of the outputlight emitted toward the direction G for the viewpoint 180 having theangle “g” is measured by using the light receiving device 1179, and thecolor difference can be obtained based on this measurement. Further,even if the hologram screen has the color distribution in which thecolor difference is 0.1 or less, there is no unevenness of color on thereproduced image.

[0258] (Third Embodiment)

[0259] As shown FIGS. 10 to 14, the explanation will be given for thehologram screen which was produced by the photosensitive material havingthe inclined thickness distribution below.

[0260] First, the exposure optical system in FIG. 10 is explained below.In the exposure optical system 2, the laser beam (wavelength: 14.5 nm,Ar laser beam) emitted from the laser oscillator 1210 is splitted by thebeam splitter 1221 for two laser beams 1212 and 1213. The splitted beam1212 becomes divergence light through the object lens 1222, and thedivergence light is irradiated to the parabolic mirror 1214 so that itis possible to obtain the parallel light 1215. The parallel light 1215is transmitted through the light diffusion body 1216 which is made of aground glass so that the diffusion light can be obtained and used as theobject light 1217. On the other hand, another splitted light 1213 istransmitted through the object lens 1221 so that the reference light1218 can be obtained.

[0261] As shown in FIGS. 10 and 11, the object light 1217 and thereference light 1218 are irradiated to the photosensitive material 1220in order to form the interference fringe. Accordingly, the interferencefringe can be recorded on the photosensitive material 1220. As a result,the light diffusion body 1216 is recorded on the photosensitive material1220. When the image light 1120 is irradiated on the hologram screen 11,the light diffusion body 1216 is reproduced, and it is possible todiffract and scatter the image light 1120 on the hologram screen 11 whenthe diffusion light is emitted from the light diffusion body 1216.

[0262] In this case, in the exposure optical system 2 shown in FIGS. 10and 11, the incident angle θ₀ and the incident distance R₀ of thereference light 1218 to the center 1229 on the photosensitive material1220 is the same as the incident distance F₀ and the incident angle f₀of the image light 1120 to the center 1109 on the hologram screen 11.

[0263] The hologram screen 11 was made by exposing two kinds tophotosensitive materials using the exposure optical system 12. Thephotosensitive material of the sample 1 has the thickness distributionshown in FIG. 12. That is, the sample 1 has the inclined thickness,i.e., the thickness is different between one end and the other end ofthe photosensitive material. In FIG. 12, the abscissa is the distancefrom the center location of the photosensitive material. A minus side ofthe center represents an irradiation side, and a plus side represents aside opposite to the irradiation. On the other hand, in FIG. 12, thephotosensitive material of the comparing sample C1 has a uniformthickness.

[0264] The sample 1 and the comparing sample C1 were provided to theexposure optical system 2 shown in FIGS. 10 and 11, and the hologramscreen was made by irradiating the reference light 1218 and the objectlight 1217. Further, both the sample 1 and the comparing sample C1 havethe size of 300 mm×400 mm, and made of a photopolymer by Dupont Co.,Ltd. Further, the light diffusion body 1216 is made of a ground glasshaving the scattering angle of 360, and the intensity ratio of thereference light 1118 and the object light 1117 at the center of thephotosensitive material 1220 is 4 (=I_(R)/I_(O)). The incident angle θ₀of the reference light at the center of the photosensitive material 1220is the same as the incident angle f₀ of the image light, i.e., 30°, whenreproducing the image on the hologram screen (see FIG. 1).

[0265] The measurement of the spectral characteristic of the hologramscreen having the sample 1 and the comparing sample C1 is explained withreference to FIG. 3 below. As shown in FIG. 3, the white light 1130 isirradiated to the hologram screen 11. In three kinds of output lightsdiffracted at the points (1), (2) and (3), in particular, the spectralcharacteristic of the output light to the direction perpendicular to thesurface at the output side on the hologram screen 11 was measured byusing the light receiving device 1179. The incident angle of the whitelight 1130 is the same as the incident angle of the image light whenreproducing the image on the hologram screen. Further, the point (2) isthe center of the hologram screen 11. The spectral characteristic of thesample 1 was shown in FIG. 13, and that of the comparing sample Cl wasshown in FIG. 14. As shown in FIG. 13, the sample 1 has approximatelythe same peak wavelength on the curves (1), (2) and (3), and thesespectral characteristics are arranged each other. Further, thechromaticity coordinate u′v′ was obtained in accordance with the abovemeasurement so that the maximum color difference became 0.015.

[0266] When the hologram screen was observed based on the sameconditions as the first embodiment, unevenness of color was not almostrecognized in this embodiment. As mentioned above, by using thephotosensitive material having the inclined thickness distribution, itis possible to obtain the hologram screen having no unevenness of colorwhen reproducing the image. Further, as shown in FIG. 14, in thehologram screen having the comparing sample C1, the spectralcharacteristic (1) has the high peak of the efficiency at the wavelengthof a blue area, and the spectral characteristic (3) has the high peak atthe wavelength from a green area to a yellow area. The spectralcharacteristic (2) has the high peak at the vicinity of intermediateefficiency of (1) and (3).

[0267] The maximum color difference was measured by using the samemethod as the first embodiment. As a result of measurement, the maximumcolor difference became 0.073. Further, the unevenness of color wastested for this hologram screen. As a result, all observers answeredlevel 3 or more (i.e., the observer slightly do mind the unevenness ofcolor.). As is obvious from the above, the difference in the spectralcharacteristic at each portion of the hologram screen is caused ofunevenness of color when reproducing the image on the hologram screen.

[0268] (Fourth Embodiment)

[0269] As shown in FIGS. 15 to 19, the explanation will be given to thehologram screen which was made by the photosensitive material previouslyirradiated by the ultraviolet before exposure. In this case, theexposure optical system is the same as the third embodiment shown inFIG. 10. Further, the ultraviolet is irradiated on the photosensitivematerial so as to become gradually strong from one incident side to theother incident side as explained in detail below.

[0270] The above irradiation method of the-ultraviolet will be explainedin detail with reference to FIGS. 15 and 16. In FIG. 15, thephotosensitive material 1220 has the size of 300 mm×400 mm, and was madeof the photopolymer by Dupont Co., Ltd. The photosensitive material wasprovided on a slanted base 132, and an ultraviolet lamp 130 was providedto the upper location of the end of the slanted base 132 as shown inFIG. 15. The ultraviolet was irradiated to the photosensitive material1220 in such a way that the amount of the ultraviolet has thedistribution shown in FIG. 16. In this case, P shown in FIG. 15 is thedistance on the photosensitive material 1220, and corresponds to theabscissa of the graph shown in FIG. 16. That is, the amount of theultraviolet is small at one end of the photosensitive material 1220(approximately, 1.5 μj/cm²), and becomes large at the other end of thephotosensitive material 1220 (approximately, 3.5 μj/cm²).

[0271] As a result, an amount of absorption of the photosensitivematerial at the wavelength 414 nm was changed in accordance with thedistance P as shown in FIG. 17. In FIG. 17, the dotted line representsthe amount of the absorption before irradiation of the ultraviolet, andthe solid line represents after irradiation. The hologram screen wasmade after irradiation of the ultraviolet using the exposure opticalapparatus, as well as the third embodiment. In this case, variousconditions were determined as follows. That is, the scattering angle ofthe light diffusion body is 36°, the intensity ratio of the referencelight and the object light at the center of the photosensitive materialis 4 (=I_(R)/I_(O)), and the incident angle of the reference light atthe center is the same as the incident angle of the reference light,i.e., 30°, when reproducing the image on the hologram screen.

[0272] In the above conditions, the color difference between optionaltwo points (see the first embodiment) on the hologram screen was 0.023in maximum. Further, unevenness of color cannot be recognized whenobserving the hologram screen in accordance with the same conditions asthe first embodiment. Further, the peak wavelength in the spectralcharacteristic of the hologram screen was measured at the points (1),(2) and (3) in FIG. 3. Further, when the ultraviolet is not irradiated,the spectral characteristic was also measured in order to compareirradiation with non-irradiation of the ultraviolet. As a result, in thecase of the spectral characteristic (1), the wavelength 460 nm (in thecase of non-irradiation) was 462 nm (in the case of irradiation). In thecase of the spectral characteristic (2), the wavelength 510 nm (in thecase of non-irradiation) was 480 nm (in the case of irradiation). In thecase of the spectral characteristic (3), the wavelength 510 nm (in thecase of non-irradiation) was 488 nm (in the case of irradiation).

[0273] The relationship between an amount of irradiation of theultraviolet and reduction of sensitivity is explained using thephotopolymer made by Dupont Co., Ltd., below. FIG. 18 shows an amount oflaser having wavelength 514.5 nm. A coloring matter mixed with thephotopolymer absorbs the light so that the polymerization reaction ofthe monomer is started. Accordingly, when the photopolymer is exposed bythe laser having wavelength 514.5 nm in the production of the hologramscreen, the amount at this wavelength represents the sensitivity of thephotopolymer.

[0274] As shown in FIG. 18, when the hologram screen was produced byexposing the photopolymer in which the sensitivity was reduced due toirradiation of the ultraviolet, it was clarified that the peakwavelength of the spectrum characteristic was shifted to the shortwavelength side due to increase of the amount of the energy as shown inFIG. 19. Accordingly, when showing the spectral characteristic of thehologram screen s shown in FIG. 14 by previously irradiating theultraviolet on the photosensitive material, it is possible to shift thespectral characteristic to the short wavelength side since the amount ofthe energy of the ultraviolet become large at the location (see (3))apart from the incident side of the reference light. That is, it ispossible to bring the spectral characteristics (2) and (3) close to thatof (1).

[0275] Accordingly, by preferably setting the amount of the energy ofthe ultraviolet to be irradiated, it is possible to arrange the spectralcharacteristic of the hologram screen, and to reduce unevenness of coloruntil the grade which is difficult to recognize the same. Further, ifthe spectral characteristic is intended to bring it near to that of (2),it is possible to shift the whole to the long wavelength side bychanging the incident angle of the reference light at the exposure andthe incident angle of the reproducing light at the reproduction, and bysetting the incident angle of the reference light so as to become small.In this case, it is possible to realize a predetermined effect in thedifference of the angle of 1 to 2°.

[0276] (Fifth embodiment)

[0277] As shown in FIGS. 20 to 25, in this embodiment, the hologramscreen is made by providing the incident distance of the reference lightat the optional location on the photosensitive material so as to becomeshorter than the incident distance of the image light which is inputfrom the image projector when reproducing the image on the hologramscreen. The hologram screen is made by exposing the photosensitivematerial using the exposure optical apparatus shown in FIG. 10. At thattime, as shown in FIG. 20, the incident distance R₀ of the referencelight was shortened to the distance R₀′. In this case, the distance R₀′is equal to the distance F₀ of the image light when reproducing theimage (see FIG. 1) on the hologram screen.

[0278] The performance of the hologram screen which was made byshortening the distance of the reference light was evaluated as follows.First, the exposure optical apparatus was prepared as shown in FIG. 10.In this case, the incident angle θ₀ of the reference light 1218 was setto 400, and the incident distance R₀ of the reference light was set to1700 mm. Then, the photosensitive material was exposed. Further, thephotosensitive material 1220 in which the exposure was completed, wasreplaced to new one. Further, the exposure was performed after theincident distance R₀ of the reference light was shortened to thedistance R₀ ′ as shown in FIG. 20. In this case, a photopolymer made byDupont Co., Ltd. was used as the photosensitive material 1220 having theuniform thickness distribution and the size of 300 mm×400 mm as shown bythe comparing sample C1 in FIG. 12 in the third embodiment. Further, thescattering angle of the light diffusion body was set to 36°, and theintensity ratio of the reference light 1218 and the object light 1217 atthe center of the photosensitive material 1220 was set to the value 4.Further, the incident angle f₀ of the image light of the hologram screen11 (see FIG. 1) was set to 400.

[0279] Based on the above conditions, each peak wavelength at thespectral characteristic of each hologram screen, in which the incidentdistance R₀ of the reference light was shortened, was measured as shownin FIG. 23. In FIG. 23, (1), (2) and (3) show the spectralcharacteristics of the output light at the location shown in FIG. 3. Asshown in FIG. 23, when the shortened amount of the reference lightbecomes large, the peak wavelength of each spectral characteristic (1),(2) and (3) is brought close to each other. When the shortened amountreaches −700 mm, three peak wavelengths (1), (2) and (3) approximatelycoincide with each other.

[0280] Further, FIG. 25 shows the relationship between the maximum valueof the color difference and the reduced amount of the distance betweenthe optional two points on the hologram screen. As shown in FIG. 25,when the reduced amount becomes large, the color difference becomessmall. For example, when the shortened value was −550 mm, the colordifference was 0.03 or less. Further, when the shortened value was −700mm, the color difference was 0.021 or less. As is obvious from theabove, it is possible to realize the hologram screen without unevennessof color by reducing the incident distance of the reference light.

[0281] (Sixth embodiment)

[0282] In this embodiment, as shown in FIG. 26, a lens 141 was providedbetween the object lens 1221 and the photosensitive material 1220.Accordingly, it is not necessary to shorten the incident distance of thereference light than the incident distance of the image light in theexposure optical system shown in FIG. 10. As a result, it is possible tosubstantially shorten the incident distance of the reference light whenproducing the hologram screen. As shown in FIG. 26, the reference light1218 and the object light (not shown in this drawing) are irradiated onthe photosensitive material 1220, as well as the exposure optical systemshown in FIG. 10, so as to expose the photosensitive material. In thiscase, the convex lens 141 having an f-value of 175 mm was provided atthe location of which the distance Q is 350 mm from the object lens1221.

[0283] In this structure, when the divergence light from the object lens1221 is input to the convex lens 141, this light becomes the convergencelight so that this light is focused on the light path. As an effect ofthe convex lens, it is possible to shorten the incident distance of thereference light, for example, the distance −700 mm, without change ofthe physical location of the photosensitive material 1220 and the objectlens 1221. In this embodiment, other conditions are the same as thefifth embodiment.

[0284] When the convex lens 141 is provided as mentioned above, it isnot necessary to limit the divergence point of the reference light 1218to the location of the distance 1700 mm from the center 1229 of thephotosensitive material 1220, and to freely change the location of theobject lens 1221 and the photosensitive material 1220 in accordance withthe f-value of the lens to be used. In FIG. 26, although the convex lens141 was used, it is possible to use a concave lens 142 in order toshorten the incident distance of the reference light as shown in FIG.27. Further, it is possible to utilize a cylindrical lens instead of theconvex lens and the concave lens.

[0285] The cylindrical lens is very effective in the case that the colordifference is reduced for the incident direction of the reference light,but the color difference is not reduced for the direction perpendicularto the surface of the drawing sheet (FIG. 26). This is because, when thedistance of the reference light is shortened in order to arrange thepeak wavelength at the points (1), (2) and (3) in FIG. 26, the peakwavelength is extremely shifted to the direction perpendicular to thesurface of the drawing sheet so that the color difference may beincreased.

[0286] (Seventh embodiment)

[0287] In this embodiment, in all areas on the photosensitive material,the intensity ratio of the reference light and the object light isdefined as I_(R)/I_(O)<10 (where, I_(R) is the intensity of thereference light, and I_(O) is the intensity of the object light). Thatis, as shown in FIG. 28, the hologram screen was made by changing theintensity ratio of the reference light 1218 and the object light 1217 onthe photosensitive screen 1220. Further, the hologram screen was made insuch a way that the conditions of the intensity ratio can be establishedin all areas on the photosensitive material 1220.

[0288] As other conditions, the incident angle of the reference light tothe center 1229 of the photosensitive material 1220 was set to 40°, andthe incident distance of the reference light was set to 1350 mm.Further, the photopolymer made by Dupont Co., Ltd. having the uniformthickness distribution and the size of 300 mm×400 mm, as shown in FIG.12 in the third embodiment was used as the photosensitive material 1220.Further, the light diffusion body having the scattering angle of 36° wasused. Still further, the incident angle of the image light to the centerwas set to 400, and the incident distance of the image light was set to1700 mm.

[0289] The relationship of the maximum value of the color difference andthe intensity ratio I_(R)/I_(O) between optional two points on thehologram screen is shown in FIG. 29. As shown in FIG. 29, when theI_(R)/I_(O) becomes small, the maximum value of the color differencebecomes small. For example, when I_(R)/I_(O) is 10 or less, the maximumvalue becomes 0.06 or less. Further, when I_(R)/I_(O) is 0.7, themaximum value of the color difference can be reduced by 0.013. Asmentioned above, when I_(R)/I_(O) is 10 or less, it is possible torealize the hologram screen without unevenness of color on the image.

[0290] Explanations of the Second Aspect of the Present Invention

[0291] The second aspect of the present invention relates to a methodfor producing a hologram element which can be used as a displayapparatus for displaying a still image or an animated image byirradiating the image light.

[0292] The object light 214 and the reference light 216 are irradiatedon the photosensitive material 230 and exposed by using an exposureoptical system 29 as shown in FIG. 52. A hologram screen formed by thehologram element, as shown in FIG. 35, can be applied to the displayapparatus 235 for displaying the still image or animated image byirradiating the image light 2390 from the irradiating apparatus 39.

[0293] On the other hand, in general, a large size screen has beendesired as the display apparatus in order to utilize as an advertizingboard in a showroom. However, it is very difficult to produce a largesize hologram screen by using the exposure optical apparatus 29 asmentioned above in once projection of the image. That is, although it isnecessary to provide a large size photosensitive material in order toproduce the large size hologram screen, a lot of time is required forexposure of the photosensitive material. Further, since it is necessaryto irradiate the reference light onto the whole of the large sizephotosensitive material, the intensity of the reference light becomesweak so that the exposure time also becomes long. As a result, theexposure time may exceed the time for stably maintaining theinterference fringe on the photosensitive material so that theproduction efficiency of the hologram element deteriorates.

[0294] Further, it is necessary to utilize the large size lightdiffusion body in order to obtain the object light which can beirradiated on the whole of the large photosensitive material. However,it is very difficult to produce the large light diffusion body. Stillfurther, since the object light which is obtained by the large sizephotosensitive material is formed by the diffusion light which isdiffused in the wide extent, the light intensity becomes weak so thatthe exposure time for exposing the photosensitive material becomes long.

[0295] In order to resolve the above-mentioned problems, the inventorhas proposed two methods, i.e., (1) a method of resolving problems byreducing a size of the photosensitive material, and (2) a method ofresolving problems by reducing a size of the light diffusion body, inJapanese Unexamined Patent Publication No. 9-349572.

[0296] In the method (1), as shown in FIG. 34, a plurality of hologramscreens 231 to 234 are integrated two-dimensionally by using a suitableadhesive material in order to obtain the large hologram screen 23. Inthis case, four hologram screens are separately produced and coupledeach other.

[0297] In the method (2), as shown in FIGS. 31 and 32, a plurality ofmirrors 221 to 224 are provided perpendicularly to the light diffusionbody 220. In this case, since the light diffusion body 220 is imaged onthe plurality of the mirrors 221 to 224, it is possible to obtain theobject light having the large extent as well as the large diffusionbody.

[0298] There is a problem, however, in the method (2). That is, whencombining the method (1) with the method (1), it is impossible toperform sufficient recording on the photosensitive material from thelight diffusion body imaged on the mirror, and there is the differencein brightness between an area directly recorded by the light diffusionbody and another area indirectly recorded by the light diffusion bodythrough the mirror. As a result, depending on the angle observed by theobserver, the image on the hologram screen becomes partially dark, andcolor are changed. Further, a boundary line between the diffusion bodyand the mirror may be imaged on the hologram screen so that the observermay has uncomfortable feeling when observing image on the hologramscreen.

[0299] According to the producing method of the second invention, it ispossible to display uniform and bright image on the display apparatus,and to observe the image in the broad angle without changes ofbrightness and color eye.

[0300] According to the invention defined in claim 13, the hologramelement is formed by interferencing the reference light with the objectlight transmitted through the light diffusion body, and by recording thelight diffusion body on the photosensitive material in the opticalsystem. There is a mirror arranged approximately perpendicularly to thelight diffusion body on the light diffusion body. Further, a rotationalcenter is defined by an axis intersected perpendicularly to the centerof the photosensitive material, and the photosensitive material and thelight diffusion body are arranged with the angle θ.

[0301] According to the second aspect of the present invention,polarized direction of laser light is determined by eitherP-polarization or S-polarization which are inclined from θ−5 to θ+5.That is, as the feature of the present invention, either P-polarizationor S-polarization having the polarized direction in the range of theangle θ−5 to θ+5 is used as the laser light.

[0302] When the polarized direction is out of the this range, the lightdiffusion body which is imaged on the mirror becomes dark, the imagebecomes partially dark depending on the observer's angle.

[0303] The operation of the second aspect of the present invention willbe explained below. The mirror is arranged perpendicular to the lightdiffusion body used in the present invention (see FIGS. 31 and 32). Inthe recording of the light diffusion body on the photosensitivematerial, the object light is irradiated from the light diffusion body,a part of the object light is irradiated directly on the photosensitivematerial, but other object light is irradiated on the photosensitivematerial after reflected by the mirror. Accordingly, the incident angleof the object light on the photosensitive material becomes broad byusing the above compact light diffusion body so that it is possible toobtain the object light having broad extent as well as the large sizelight diffusion body.

[0304] Further, when producing the hologram element, the light diffusionbody and the photosensitive material are rotated with the same angle θaround the axis which is intersected perpendicularly to the center pointof the photosensitive material and used as the center of the rotation.

[0305] In this case, when the large size hologram screen 3 shown in FIG.34 is produced by one hologram element, it is necessary to havearrangement relationship among the photosensitive material 290, thelight diffusion body 294 and the reference light 292 shown in FIG. 37.Further, as shown in FIG. 34, when one large size hologram screen isformed by coupling four hologram elements 231 to 234, there is thearrangement relationship between the photosensitive material 2901necessary for producing one hologram element 231 and the light diffusionbody 2941 used for the photosensitive material 2901.

[0306] In FIG. 37, there is the divergence point 2920 of the referencelight 292 on the plane AA formed by the center line X passing the centerC and the axis 2905 passing perpendicularly to the center point C of thephotosensitive material 290. In this case, the divergence point 2920becomes the object lens 2105 in the optical system 29. Further, in FIG.37, the straight line X1 passing the center point C1 and parallel to theline X is provided on the photosensitive material 2901. Further, theplane AA1 (not shown) including the line X1 is provided parallel to theplane AA. In this case, there is no divergence point 2920 on the planeAA1.

[0307]FIG. 38 shows the configuration observed from the Z-axis of FIG.37. As shown in FIG. 38, it is necessary to irradiate the referencelight 292 toward the center point C1 with the angle θb for the centerline X (plane AA). That is, when producing the hologram element 231which partially forms the large size hologram screen 23, it is necessaryto irradiate the reference light 292 with a predetermined angle.

[0308] In FIG. 38, there is an axis passing the center point C1 andperpendicular to the drawing sheet. The photosensitive material 2901,the light diffusion body 294 and the reference light 292 are rotatedwith the angle θb to the right direction around the above axis used asthe rotational center, as shown in FIG. 39. When these are rotated withthe angle θb, the divergence point 2920 of the reference light 292 ismoved to the plane AA1.

[0309] Accordingly, when producing the hologram element, the lightdiffusion body and the photosensitive material are rotated with the sameangle θ as mentioned above. Further, the reference light is irradiatedto the photosensitive material on the same horizontal plane as theobject light. When θ=θb, it is possible to easily and safety produce thehologram element since the optical system for exposing thephotosensitive material can be easily provided.

[0310] In the present invention, either the P-polarization or theS-polarization having the polarization direction in the extent from theangle θ−5 to θ+5 is used as the laser light, as shown in FIG. 33. Inthis case, FIG. 33 show the configuration observed from the directionshown by the arrow “a” in FIG. 32. It is assumed that the laser light ofthe S-polarization having the polarized direction shown by the arrowline LO in FIG. 40 is irradiated to the light diffusion body. In thiscase, the polarized direction is perpendicular to the mirrors 223 and224 as shown in FIG. 40.

[0311] There are the light diffusion body 220 and the mirrors 221 to 224provided thereto, which are rotated with the angle θ around the axisintersected perpendicularly to the center point of the light diffusionbody 220 and used as the rotational center as shown in FIG. 41. In thiscase, since the polarized direction of the laser light is not changedand directed by the arrow line LO, the perpendicular direction (i.e.,S-polarization) is maintained.

[0312] The laser light is transmitted through the light diffusion body220 as shown in FIG. 42, and reflected by the mirror 222 so that theobject light Ob is formed. In this case, the polarized direction of theobject light is considered. As shown in FIG. 42, the polarized directionof the object light Oa, which transmits the light diffusion body 224 andreaches the mirror 222, is shown by the arrow line La. However, sincethe mirror 222 is inclined with the angle θ, the polarized direction ofthe object light Ob which is reflected by the mirror 222 is shown by thearrow line Lb. That is, this polarized direction is inclined with 2×θfor the polarized direction before reflection.

[0313] In this case, the polarized direction of the reference lightwhich reaches the photosensitive material is the same as that of thelaser light and has the S-polarization. Accordingly, in the object lightOb after reflection, the object light which can contribute to form theinterference fringe with the reference light, as shown in FIG. 43, isonly the same direction component as the arrow line Lc which indicatesperpendicular component of the arrow line Lb. That is, the substantialintensity of the object light reflected by the mirror deteriorates sothat it becomes weaker than the intensity of the object light which wasdirectly input to the photosensitive material after transmitting thelight diffusion body. As a result, the efficiency of the interferencefringe becomes lower than the efficiency of the interference fringeformed by the object light which was directly input to thephotosensitive material.

[0314] As mentioned above, the sufficient recording to thephotosensitive material cannot be performed from the light diffusionbody which was imaged on the mirror, and the difference in brightnessoccurs between the recorded area directly imaged by the light diffusionbody and the recorded area indirectly imaged by the light diffusion bodythrough the mirror. As a result, there is a problem in which the imagebecomes partially dark on the hologram screen, depending on theobserving angle from the observer.

[0315] In the present invention, as mentioned above, the P-polarizationor the S-polarization having the polarized direction in the range fromthe angles θ−5 to θ+5 is used as the laser light. Accordingly, it ispossible to provide the state of the P-polarization or S-polarizationfor the light diffusion body and the mirror inclined with the angle θ.That is, in the present invention, the object light is reflected by themirror so that the large change of the polarization direction isprevented, and it is possible to prevent a partially dark image whichdepends on the observing angle from the observer.

[0316] In the S-polarization, for example, in FIG. 33, the polarizeddirection of the laser light is inclined with the angle θ so that thepolarized direction of the laser light becomes parallel to the mirrors221 and 222. The laser polarization is called “S-polarization havingangle θ” in the present invention, and the polarized direction (i.e.,parallel to mirrors 23 and 24) perpendicular to the S-polarization iscalled “P-polarization having angle θ”.

[0317] The laser light used in the present invention is generated by aknown laser oscillator, and it is possible to prepare the P-polarizationor the S-polarization having the polarized direction in the range fromthe angles θ−5 to θ+5 by providing a half-wave plate for the laseroscillator. The wavelength plate is an optical element which can changethe polarization state of the optical wave by providing the phasedifference based on the difference in the refractive index for thepolarized component of the optical crystal having the opticalanisotropy. Further, the half-wave plate is an optical element which canchange the polarized direction of the linear polarization.

[0318] When the linear polarized light is irradiated with the angle θfor the optical axis of the half-wave plate, the polarized direction ofthe output light has the direction which is inclined with the angle 2×θfor the incident light. Further, the angle θ can be changedcontinuously. As a result, it is possible to easily obtain the laserlight having the desired polarized direction. Further, it is possible toutilize the wavelength plate having the above function utilizing theoptical anisotropy of the optical crystal as the half-wave plate.Further, it is possible to utilize a Fresnel rhomb plate which utilizesthe total reflection in the optical element as the half-wave plate.Particularly, the Fresnel rhomb wavelength plate can be used for thehigh power laser light, and can be used widely since the usablewavelength band of the laser light is wide. Accordingly, the Fresnelrhomb wavelength plate can be preferably used as the half-wave plate.

[0319] Further, as a method for integrating a plurality of hologramelements, for example, there are two representative methods, i.e., onebeing a method for bonding the hologram element to a predetermined film(for example, a PTE film) with an adhesive material, and the other beinga method for bonding the hologram element to a large glass plate using atransparent and optical adhesive material. Further, it is possible toutilize the light which is generated by an argon (Ar) laser oscillatoras the laser light. Further, it is possible to utilize the photopolymer,or the dichromated gelatin, etc., as the photosensitive material. Stillfurther, it is possible to utilize ground glass or opal glass as thelight diffusion body.

[0320] According to the second invention, it is possible to produce thetransparent hologram element -in order to form the hologram screen. Inthis case, it is possible to obtain the transparent hologram screen byusing a transparent adhesive material. As a result, the displayapparatus having the hologram screen can be effectively used as, forexample, not only an advertising board for a show window, showroom,etc., in a shopping center, but also as a transparent screen which cansee through an opposite person when the display apparatus is not used,in a ticket window.

[0321] Preferable embodiments in the second embodiment will be explainedin detail with reference to the attached drawings.

[0322] (First Embodiment)

[0323] The first embodiment will be explained in detail with referenceto FIGS. 30 to 35. In this embodiment, as shown in FIG. 34, fourhologram elements 231 to 234 are integrated two-dimensionally by using apredetermined adhesive material in order to form the hologram screen 23.

[0324] In the optical system shown in FIG. 30, the laser light 211 issplitted to two separated lights 211 and 212. The separated light 212becomes the object light 214 after transmitting the light diffusion body220, and the other separated light 215 becomes the reference light 215.The object light 214 and the reference light 216 are irradiated on thephotosensitive material 230 in the same horizontal plane, and the lightdiffusion body 220 is recorded on the photosensitive material 230 sothat each of hologram elements 213 to 234 is produced. After thehologram elements 231 to 234 were integrated, the large hologram screen23 is produced as shown in FIG. 34.

[0325] When producing the hologram elements, the mirrors 221 to 224 areprovided perpendicularly to the light diffusion body 220 thereon asshown in FIGS. 30 to 32. Further, as shown in FIG. 32, the lightdiffusion body 220, mirrors 221 to 224 and the photosensitive material230 are rotated with the same angle θ around the axis 2000 which isintersected perpendicularly to the center point of the photosensitivematerial 230 and used as the rotational center, and the S-polarizationhaving the polarized direction of the angle θ is used as the laser light211. The rotational angle θ is different in each hologram elements 213to 234.

[0326] In the optical system 21 shown in FIG. 30, the wavelength plate2101 is provided between the laser oscillator 210 and the beam splitter2102, and the S-polarization having the polarized direction with theangle θ can be obtained when the laser light is passed through thewavelength plate 2101. Further, the wavelength plate 2101 is formed bythe Fresnel rhomb half-wave plate. Further, the laser oscillator 210 isformed by the Ar laser oscillator. Still further, the laser lightemitted from the laser oscillator 210 and passed through the wavelengthplate 2101 is split by the beam splitter 2102 into two beams so that theseparated lights 212 and 215 can be obtained.

[0327] After one separated light 212 became the divergence light 2130 bythe object lens 2103, and the divergence light 2130 becomes the parallellight 213 by the parabolic mirror 2104. Further, the diffusion light isformed after the parallel light 213 was passed through the lightdiffusion body 220, and becomes the object light 214. Further, whenanother separated light 215 is passed through the object lens 2105 sothat the reference light 216 can be obtained.

[0328] As shown in FIGS. 31 and 32, a light diffusion unit 22 is formedby the square and plate-like light diffusion body 220 made of, forexample, cloudy glass, and the mirrors 221 to 224 providedperpendicularly to the light diffusion body 220. Further, in the lightdiffusion unit 22, the length of the mirror 221 is shorter than those ofmirrors 222 to 224 in order to form a space which delivers the referencelight 216. Further, the mirrors 221 to 224 are formed by adielectric-multilayer film, and there is no slippage of the phase of thereflection light in each mirror. Further, it is possible to use ametal-vapoured film as the mirror. In this case, the mirror is arrangedin such a way that the surface thereof is faced inside.

[0329] As shown in FIG. 33, the light diffusion unit 22 is arranged tothe right direction with the angle θ. On the other hand, the lightdiffusion unit 22 is arranged perpendicularly to the light diffusionbody 220 so that the light diffusion body 220 is arranged with the angleθ. Further, although the drawing is omitted, the photosensitive material230 is arranged with the same angle. Still further, the polarizeddirection of the parallel light 213 which is input to the lightdiffusion body 220 is the direction shown by the arrow line L in FIG.33. That is, the polarized direction is directed to the angle θ.

[0330] When the object light 214 and the reference light 216 areirradiated on the photosensitive material 230, the interference fringeformed by these lights is recorded on the photosensitive material 230.As a result, the light diffusion body 220 is recorded on thephotosensitive material 230 so that the hologram screen is formedthereon.

[0331] Next, the hologram elements 231 to 234 each having width acrosscorners of twenty inches were prepared in accordance with theabove-mentioned method as explained below. In this case, the rotationalangle θ is different each other in four hologram screens. For example,in FIG. 5, when the image light 2390 is irradiated with the angle 35° atthe center of the hologram screen 23, i.e., at the center of thewidth-across-corners of forty inches in the hologram screen 23, thehologram element 231 is 15° to the right direction, the hologram element232 is 15° to the left direction, the hologram element 233 is 11° to theright direction, and the hologram element 234 is 11° to the leftdirection.

[0332] The method for coupling the hologram screens 231 to 234 will beexplained in detail below. As shown in FIG. 36A, the hologram element231 is laminated on the glass substrate 241, and the transparent PETfilm 242 (a size of twenty inches) with the adhesive material is bondedby using a roller 240 on the glass substrate 241. After the peripheralportion of the hologram element 231 and the transparent PET film 242which exceed the size of twenty inches was cut off, the glass plate 241is removed.

[0333] After the above steps, the transparent PET film 242 was bonded tothe remaining three hologram screens 232, 233 and 234. Further, as shownin FIG. 36B, after the four hologram screens 231 to 234 were closelyarranged so as not to provide a gap between hologram elements, thetransparent PET film 243 with the adhesive material and being fortyinches wide was bonded on the surface of the hologram element 232 usingthe roller 240. As a result, it is possible to obtain the large sizehologram screen 23 having the width across corners of forty inches.

[0334] Next, the display apparatus 235 using the hologram screen 23according to the second invention will be explained in detail below.

[0335] As shown in FIG. 35, the image projector 239 is arranged at theback and upper portion of the hologram screen 23. The image projected bythe image projector 239 is focused on the hologram screen 23 so that theobserver 238 can observe the image on the hologram screen 23. That is,when the image light is irradiated on the hologram screen, the lightdiffusion body 220 which is recorded on the hologram elements 231 to 234is reproduced, and the image light is diffracted and diffused on thehologram elements 231 to 234. As a result, the hologram screens 231 to234 can be functioned as the hologram screen.

[0336] Further, the hologram elements 231 to 234 are formed by thetransparent material, and the adhesive material for these hologramelements is the transparent PET films 242 and 243. Accordingly, theobserver can observe the background 2381 as well as the image on thehologram screen.

[0337] The operation of this embodiment will be explained in detailbelow. As shown in FIGS. 31 and 32, the mirrors are providedperpendicularly to the light diffusion body. In the exposure process ofthe photosensitive material, the separated light is irradiated to thelight diffusion body, and the object light is output from the lightdiffusion body. In this case, a part of the object light directlyreaches the photosensitive material, and the remaining object lights areonce reflected by the mirror and reaches the photosensitive material.

[0338] Accordingly, the incident angle of the object light to thephotosensitive material becomes wide so that it is possible to obtainthe object light having the same extent as the large light diffusionbody by using the small light diffusion body according to thisembodiment.

[0339] As mentioned above, four hologram elements are producedseparately, and integrated to form the hologram screen. When producingthe hologram screen, the light diffusion body and the photosensitivematerial are rotated by the same angle θ. In this case, the angle θ isdifferent in each of four hologram elements. Further, the referencelight and the object light are irradiated to the photosensitive materialon the same horizontal plane.

[0340] Further, in this embodiment, the S-polarization having thepolarized direction of the angle θ as the laser light. Accordingly, itis possible to set the polarized direction of the object light to theS-polarization for the light diffusion body and mirrors which areinclined by the angle θ. That is, according to this embodiment, since itis possible to prevent large change of the polarized direction due tothe reflection of the object light by the mirror, it is possible toprevent a partially dark portion on the hologram screen and a partiallydark image which occurs depending on the observing angle by theobserver. As a result, it is possible to display uniform and brightimage on the hologram screen and to observe the image from the wideangle without change of the brightness and the color eye.

[0341] (Second Embodiment)

[0342] The second embodiment according to the second aspect of thepresent invention will be explained with reference to FIGS. 44 to 51below. In the following explanations, the producing method was comparedbetween two kinds of the S-polarization used as the laser light, i.e.,one S-polarization having the polarized direction of the angle of 15°according to the first embodiment, and the other S-polarization havingthe polarized direction of the angle of 0°. Two kinds of the hologramscreen 23 were prepared in accordance with the above two kinds of theS-polarization.

[0343] As shown in FIG. 44, the white image was projected by the imageprojector 239 from the upper and slant location to the hologram screen23. The observer 238 observed the hologram screen 23 with the naked eyefrom the upper location of the angle 15°. As a result of observation,the whole of the screen becomes uniformly white on the hologram screen23 according to the first embodiment. On the other hand, in the case ofthe S-polarization having the polarized direction of the angle of 0°,there was unevenness of brightness, i.e., bright portion “n” and darkportion “m”, on the hologram screen as shown in FIG. 45.

[0344] That is, in FIG. 45, a boundary line was clearly imaged betweenthe bright portion “n” and the dark portion “m” on the hologram screen23, and the boundary line was located at the height of 60 mm from thebottom of the hologram screen. In this case, the dark portion “m” isconsidered as the range in which the light diffusion body was recordedby the object light which was reflected by the mirror. AS is obvious,the above hologram screen corresponds to the hologram element 234 shownin FIG. 34.

[0345] In order to investigate in detail, a color brightness meter wasarranged at the location of the observer's eye 238 as shown in FIG. 44,and the brightness and the chromaticity (u′, v′) were measured at sixpoints (see, six dots in FIG. 45) for the CIE 1976 chromaticitycoordinate (u′, v′) and JIS STANDARD Z-8729. The results of measurementwere shown in FIGS. 46 to 51.

[0346] In FIGS. 46 to 51, the “position” on the abscissa is the heightfrom the bottom of the hologram screen 23, and the measuring points islocated at the distance of 200 mm from the left end of the hologramscreen 23. As is obvious from FIGS. 46 to 51, in the hologram screen 23according to the present invention, it is possible to eliminate theboundary line between the mirror and the light diffusion body, and toprovide a screen having no difference in brightness and chromaticity.

[0347] Explanations of the Third Aspect of the Present Invention

[0348] Before describing the preferred embodiments, a conventional artand its problem are explained below. For example, a known hologramscreen formed by the hologram elements has been disclosed in theJapanese Examined Patent Publication No. 52-12568. The above hologramscreen is formed by the hologram element and the substrate forreinforcement in which the hologram element is bonded. As shown in FIG.56, the image light 331, which is irradiated by the image projector 33and formed by the still image or animated image, is scattered anddiffused on the hologram element so as to form the output light 332. Theoutput light 332 reaches the observer 35 so that the image isreproduced. The hologram element can be produced by irradiating thereference light 3118 and the object light 3117 on the photosensitivematerial 3120 in the exposure optical system 310 shown in FIG. 55.

[0349] However, in the conventional hologram screen, the reproducedcolor tone of the image is different from the color tone of the imagelight 331 which is generated from the image projector 33. For example,the reproduced image becomes slightly green so that the quality of imagedeteriorates. This is because, as shown in FIG. 61, there is a peak at aparticular wavelength range on the hologram spectrum in the hologramscreen.

[0350] In the above mentioned document, i.e., JPP-52-12568, the laseroscillator which can generates red, green and blue light is provided inthe exposure optical system 310, and one photosensitive material 3120 ismultiply exposed by the red, green and blue lights, in order to resolvethe above problem. Further, there is another method which has beenalready disclosed. That is, according to this method, the red, green andblue laser lights are separately used for exposing each photosensitivematerial, and three photosensitive materials each of which was exposedby the red, green and blue lights, are laminated each other so that onehologram element is produced.

[0351] In this method, however, it is necessary to prepare at leastthree laser oscillators each of which can generate the red, green andblue laser lights, or each of which can generate three kinds ofwavelength. As a result, there is a problem in which a cost of theexposure optical system becomes high and production cost of the hologramscreen is increased. Further, although it is necessary to arrangebalance of each intensity of the red, green and blue laser lights, it isvery difficult to perform this arrangement, and a lot of time isrequired in the manufacturing process.

[0352] Further, in this method, since the red, green and blue laserlights are used for exposing the photosensitive material, it is verydifficult to arrange characteristic balance (i.e., RGB balance) betweenhologram elements each of which is recorded by the red, green and bluelaser lights. As a result, strong unevenness of color partially occurson the hologram screen so that color of the image, which is reproducedon the hologram screen, becomes worse.

[0353] Still further, in the above hologram element, three kinds oflight diffusion bodies are recorded by the red, green and blue laserlights. As a result, an external light is easily diffracted andscattered in this hologram screen so that the hologram element becomescloudy. The transparency becomes worse on the cloudy hologram element sothat appearance also becomes worse.

[0354] The third aspect of the present invention aims to solve the aboveproblems. That is, the third invention has superior color reappearanceon the hologram screen. Further, when producing the hologram elementwhich forms the hologram screen, it is possible to utilize either asingle laser light, or two color (red and green) laser lights. Stillfurther, it is possible to provide the hologram screen having a lowproducing cost and a simplified producing process, as explained indetail below.

[0355] According to the invention defined in claim 14, the image light,which is irradiated from the image projector, is scattered and diffusedon the hologram screen so that the output light is obtained and theimage is reproduced on the hologram screen.

[0356] The method of producing the hologram element is characterized inthat the half bandwidth of the hologram spectrum is 100 nm or more, and

[0357] the diffusion light obtained by the light diffusion body in whichthe light diffusion angle is large, is used as the object light, and thenon-diffusion light is used as the reference light; and both lights areirradiated on to the photosensitive material in order to form theinterference fringe thereon.

[0358] The hologram spectrum is defined as the intensity (i.e.,efficiency) for each wavelength of the output light which is scatteredand diffused on the hologram screen (see FIGS. 53 and 61). The abscissais the wavelength, and the ordinate is the efficiency. Further, theefficient is a ratio of the intensity of the output light and theintensity of the incident light.

[0359] As shown in FIG. 61, in the hologram spectrum, the wavelength atthe extreme value of the efficiency is the peak wavelength, and thewidth of the peak at the location of a half of the extreme value isdefined as the half bandwidth. When the half bandwidth becomes small,the peak of the hologram spectrum becomes sharp. When the half bandwidthbecomes large, the peak of the hologram spectrum becomes gentle.Further, the shape of the hologram spectrum becomes flat when the halfbandwidth is large, and the efficiency for each wavelength becomesuniform.

[0360] As mentioned above, in the third aspect of the present invention,the half bandwidth of the hologram spectrum is 100 nm or more.Accordingly, it is possible to obtain the output light by scattering anddiffusing without emphasis of the particular wavelength of the imagelight, and to obtain the hologram screen which can reappear clearly thecolor tone of the image light (see FIGS. 62 and 63).

[0361] Further, as mentioned above, the object light is prepared by thelight diffusion body in which the light diffusion angle is large, thescattering extent becomes large. Since the object light is irradiated,the half bandwidth of the hologram spectrum of the hologram elementbecomes 100 nm or more (see FIGS. 57 and 58).

[0362] Further, since the hologram element is prepared by the laserlight having the monochrome or two colors (i.e., red and green), thereis no problem mentioned in the conventional art. That is, since thehologram element is prepared by using the exposure optical system havingthe laser oscillator which can generate the laser light having thesingle color or two colors, it is possible to realize low producingcost. Further, since it is not necessary to arrange the intensitybalance of three laser lights each having different wavelength, it ispossible to realize easy production. Further, since the hologram elementis prepared by the laser light having the monochrome or two colors,there is no scattering and diffusion due to the external light and nocloudiness on the hologram screen so that it is superior in thetransparency.

[0363] In the third aspect of the present invention, since the hologramelement is transparent, the observer can see through the back of thehologram screen when the image is reproduced. Accordingly, it is veryeffective to utilize the hologram screen of the third aspect of thepresent invention as the advertising board in the shopping center, or asthe ticket window. When the image light is not irradiated on thehologram screen, the hologram screen is merely transparent plate so thatthere is no influence for the view field.

[0364] On the other hand, when the half bandwidth of the hologram screenis 100 nm or less, since the output light is emphasized for theparticular wavelength band, the color reappearance of the image lightmay become worse. Since the half bandwidth becomes large, the hologramspectrum becomes flat and the color reappearance of the image light isimproved. However, since a human eye can recognize only visible ray, itis sufficient to set the half bandwidth to approximately 400 nm.

[0365] As the light diffusion body in which the light diffusion angle islarge, it is possible to utilize a double-faced ground glass, anone-faced ground glass, and an opal glass. Further, as a method forenlarging the light diffusion angle of the light diffusion body, forexample, there is the method for enlarging the light diffusion angle bylaminating a plurality of light diffusion bodies. Further, there is themethod for changing the scattering characteristic of the light diffusionbody itself by changing the material of the light diffusion body, orproviding the surface process on the light diffusion body. Stillfurther, it is possible to realize the large light diffusion angle bycombining lens array. In this case, by partially changing the f-value ofthe lens, it is possible to partially change the scatteringcharacteristic of the light diffusion body, and to obtain the goodscattering characteristic.

[0366] The above light diffusion angle will be explained in detailbelow. The scattering light is generated by irradiating the laser lightperpendicularly to the light diffusion body. At that time, when theintensity of the laser light which transmitted perpendicularly to thelight diffusion body is set to 100, the efficiency of the scatteringlight for each output angle is measured. In this case, the output angleat the efficiency of 50% is the light diffusion angle (see the firstembodiment).

[0367] According to the invention defined in claim 15, the lightdiffusion angle is preferable at the time when the light diffusion angleof the hologram screen is 10° or more. In this angle, it is possible toobtain the hologram screen having superior reappearance. When the lightdiffusion angle of the hologram screen is 10° or less, the scatteringextent of the object light becomes small, it is difficult to obtain thehologram element having the half bandwidth mentioned above. Further,although the color reappearance becomes good when the light diffusionangle of the light diffusion body becomes large, the brightness of theimage becomes worse so that it is difficult to watch the image.Accordingly, it is preferable to set the upper limit of the lightdiffusion angle to the angle 60°. Still further, when the hologramscreen shown in FIG. 56 is used as the display apparatus, it ispreferable to set the light diffusion angle in the range from 36° to50°.

[0368] According to the invention defined in claim 16, the halfbandwidth of the hologram spectrum is 100 nm or more. The diffusionlight obtained by the light diffusion body is used as the object light,and the non-diffusion light is used as the reference light. The objectlight and the reference light are irradiated on the photosensitivematerial having the thickness of 1 to 20 μm in order to from theinterference fringe on the hologram element. Accordingly, it is possibleto obtain the scattered and diffused output light without emphasis ofthe particular wavelength of the image light. As a result, it ispossible to realize the hologram screen which can preferably representthe color tone of the image.

[0369] As shown in FIGS. 64 and 65, and in the second embodiment, whenthe thickness of the photosensitive material is changed, the halfbandwidth is changed and the peak efficiency (the efficiency at theextreme value of the hologram spectrum) is also changed. Accordingly, itis possible to obtain the hologram element having the above mentionedhalf bandwidth by using the photosensitive material having the abovementioned thickness.

[0370] Since the above mentioned hologram element can be produced byusing laser light having the monochrome or two colors, it is possible toobtain the hologram screen having the following features, i.e., lowproducing cost, simplified producing processes, and high transparency.

[0371] On the other hand, when the thickness of the photosensitivematerial is 1 μm or less, the image becomes very dark since theefficiency of the hologram element is too low so that the observation ofthe image becomes very difficult. Further, when the thickness of thephotosensitive material is 20 μm or more, the half bandwidth is 100 nmor less so that the color at the particular wavelength becomes strongand the color reappearance becomes worse.

[0372] According to the invention defined in claim 17, the peakwavelength of the hologram spectrum is either 525 nm or less, or 585 nmor more. Further, the half bandwidth of the hologram spectrum is 100 nmor more. After the interference fringe was formed on the photosensitivematerial, the hologram element is produced by adjusting the refractiveindex of the photosensitive material.

[0373] On the other hand, there is a wavelength dependency in thesensitivity of the human eye (a relative luminosity factor (i.e., asensitivity of a human eye)) as shown in FIG. 68. That is, as shown inFIG. 68, the spectral luminous efficiency becomes the highest(strongest) when the wavelength is 555 nm.

[0374] In the third aspect of the present invention, the peak wavelengthof the hologram spectrum is either 525 nm or less, or 585 nm or more.The spectral luminous efficiency at these wavelengths are 80% for thesensitivity at the peak wavelength, i.e., 555 nm. Accordingly, it ispossible to obtain the hologram screen which can reappear the originalcolor tone of the image light without the particular wavelength on theimage light.

[0375] That is, an amount of change of the refractive index of thephotosensitive material is controlled by adjusting the processconditions from before exposure of the photosensitive material untilafter production of the hologram screen, and it is possible to realizethe hologram screen having good color-reappearance.

[0376] In the case of a volume phase type hologram which records theinterference fringe by utilizing a refractive index modulation of thephotosensitive material, although the extent of the amount of change ofthe refractive index is slightly changed depending on the photosensitivematerial to be used, the above extent is strongly depended to theexposure conditions and the process conditions before and after theexposure.

[0377] According to the Bragg's formula,

2nd·sin θ=λ  (8)

[0378] Where, “n” is a refractive index of the photosensitive material,“d” is a pitch of the interference fringe, θ is a half of an intersectedangle of the reference light and the object light, and λ is a recordedwavelength.

[0379] The pitch “d” of the interference fringe is determined at theexposure, and the amount of change An of the refractive index isdetermined after completion of all processes. The reproduced wavelengthλ′ at the reproduction of the hologram element can be expressed by thefollowing formula.

λ=2(n+Δn)d·sin θ  (9)

[0380] As is obvious from the formula (9), when the amount of change Anof the refractive index becomes large, the reproduced wavelength λ′becomes long. As mentioned above, it is possible to obtain a desiredpeak wavelength by suitably determining the amount of change Δn.

[0381] Further, a diffractive efficiency η is also changed depending onthe amount of change Δn. When the hologram is the phase andtransmittion-type hologram, the following formula is giventheoretically.

η=sin 2θ

θ=n·Δn·T/(λc)·(n ²−sin² θ)^(1/2)  (10)

[0382] As is obvious from the above formula, the diffractive efficiencyη is influenced by the amount of change Δn.

[0383] Further, there is a feature in which the diffractive efficiency ηis changed sinusoidally. In this case, however, since the above formulais given for a simple lattice (i.e., the hologram is formed by onereference light and one object light), the above formula becomes morecomplicated when there are a plurality of object lights according to thepresent invention.

[0384] The hologram spectrum is formed as shown in FIG. 84. When thereare a plurality of object lights according to the present invention, oneinclined interference fringe (i.e., a diffraction lattice) is formed forone object light (i.e., an object light having one direction). That is,it is possible to form the interference fringe which has differentinclinations (i.e., diffraction lattice) depending on the irradiateddirection of the object light at the location on the light diffusionbody. The difference of the wavelength occurs depending on thedifference of inclination when reproducing the white light.

[0385] The difference of the wavelength is shown by the solid lines inFIG. 84. The diffraction efficiency q is also changed depending on thereproduced wavelength λc. Accordingly, the efficiency is different dueto the reproduced wavelength as shown in FIG. 78. As a result, thehologram spectrum on the hologram screen is formed as shown by dottedline in FIG. 84, from a sum of the wavelengths shown by the solid lines.Accordingly, the efficiency is changed based on the amount of change Anof the refractive index for each wavelength, and the shape of thehologram spectrum, i.e., the half bandwidth is also changed.

[0386] As mentioned above, by changing the amount of change An of therefractive index of the photosensitive material after completion of allprocesses, it is possible to change not only the reproduced wavelength,but also the half bandwidth. As a result, by suitably adjusting theamount of change An of the refractive index, it is possible to adjustthe peak wavelength at the hologram spectrum to either 525 nm or less,or 585 nm or more, and to adjust the half bandwidth to 100 nm or more.Further, it is possible to adjust the half bandwidth based on theexposure intensity, the intensity ratio of the reference light and theobject light (R/O), and the heat conditions after exposure.

[0387] According to the invention defined in claim 18, after completionof the interference fringe, the heating process is provided to thephotosensitive material in the range of 80° to 150°. There are somekinds of photosensitive materials in which the refractive index can beadjusted by the heating process after exposure and the efficiency of thehologram element is increased. The amount of change An of the refractiveindex is changed based on the heating conditions.

[0388] When the heating process is performed, as is obvious from theformula (9), the peak wavelength at the hologram spectrum can beadjusted to 525 nm or less, or 585 nm or more, and the half bandwidthcan be adjusted to 100 nm or more. In this case, when the temperature atthe heating process is 80° C. or less, the image becomes dark so that itis difficult to observe the image. On the other hand, when thetemperature exceeds 150° C., and when a film is used as thephotosensitive material, the film may be deformed. Further, the color ofthe image may be worse due to color of the photosensitive materialitself.

[0389] Further, it is preferable to set the continuous heating time inthe range of 1 minute to 10 hours. When the heating time is 1 minute orless, the efficiency may be worse. On the other hand, when the heatingtime exceeds 10 hours, deformation and color may occur in thephotosensitive material, and productivity of the hologram screen becomesworse.

[0390] Still further, the optimum temperature for the heating process isdifferent in accordance with kinds of photosensitive material to beused. For example, as shown in FIGS. 80 and 81, the peak wavelength ischanged in accordance with the heating temperature and the heating time.The heating conditions is changed in accordance with kinds ofphotosensitive material, various conditions of the exposure opticalsystem, and desired characteristics so that it is very difficult touniformly determine the conditions. However, in the case of thephotopolymer made by Dupont Co., Ltd., it is preferable to set thetemperature to 100° C. to 150° C., and to set the heating time to 2hours or less.

[0391] Further, the heating method is performed within a oven so as notto contact the photosensitive material (front and back surfaces) toother parts during the heating. If the photosensitive material contactsto other parts, the difference in thermal conductivity occurs betweenthese parts so that unevenness of color brightness occur whenreproducing the image.

[0392] According to the invention defined in claim 19, a sum of anexposure intensity of the object light and the exposure intensity of thereference light is set within 0.02 to 50 mW/cm², and the object lightand reference light are irradiated onto the photosensitive material toform the interference fringe in order to provide the hologram element.

[0393] As mentioned in the formula (8), the reproduced wavelength ischanged in accordance with the amount of change An of the refractiveindex which can be changed by the exposure intensity.

[0394] For example, in the case of the polymer made by Dupont Co., ltd.,as disclosed in the document “Holographic transmission elements usingimproved photopolymerfilms” by William J. Gambogi et al., SPIE Vol.1555, Computer and Optically Generated Holographic Optics (Fourth in aSeries), 1991, pp 256-267, when the exposure intensity becomes weak, theamount of change An of the refractive index becomes large aftercompletion of all processes.

[0395] Accordingly, when the exposure intensity becomes weak, thereproduced wavelength λ′ becomes long. On the other hand, when theexposure intensity becomes strong, the reproduced wavelength λ′ becomesshort. As mentioned above, since it is possible to change the reproducedwavelength by changing the exposure intensity, it is possible to adjustthe peak wavelength at the hologram spectrum to either 525 nm or less,or 585 nm or more, by suitably adjusting the exposure intensity.Further, based on the formula (10), it is possible to adjust the halfbandwidth so as to become 100 nm or more.

[0396] When the sum of the exposure intensity is 0.02 mW/cm² or less,there are problems in which the intensity becomes very weak, and theexposure time becomes very long, so that unevenness of color may beoccur on the hologram screen. On the other hand, when the sum of theexposure intensity is 50 mW/cm² or more, there are problems in which thepeak wavelength becomes very short, and the half bandwidth becomes verynarrow, so that the color of the reproduced image becomes considerablyworse.

[0397] When the exposure intensity is constant, it is possible to changethe peak wavelength and the half bandwidth at the hologram spectrum inaccordance with the intensity ratio (R/O) of the object light and thereference light. That is, as defined in claim 20, the intensity ratio ofthe object light (O) and the reference light (R) is set to 0.1 to 30,and the object light and the reference light are irradiated onto thephotosensitive material in order to form the interference fringe. As isobvious in FIGS. 66 and 67, and the third embodiment, when the intensityratio R/O becomes small, the half bandwidth becomes large so that it ispossible obtain the superior hologram element. Since the hologramelement can be prepared by using the laser light having the monochromeor two colors (red and green), there is no problem in the thirdinvention compared to the conventional art which utilizes the multicolorlaser. That is, it is possible to realize the hologram element havinglow producing cost, easy producing process, and good transparency.

[0398] When the intensity ratio R/O is 0.1 or less, the transparency ofthe hologram screen becomes worse. However, when there is no problem ifthe transparency becomes worse, it is possible to set the intensityratio R/O to a small value in the range in which the hologram efficiencyis not decreased. In this case, the half bandwidth becomes wide, and thecolor reappearance becomes very good. Further, when the intensity R/O islarger than 30, the half bandwidth becomes small and the colorreappearance becomes worse.

[0399] According to the invention defined in claim 21, the peakwavelength of the hologram spectrum is either 525 nm or less, or 585 nmor more. The hologram element is produced by adjusting the thickness ofthe photosensitive material after completion of the interference fringe.

[0400] When the thickness of the photosensitive material is changed fromthe thickness at the exposure after completion of all processes, thepitch “d” of the interference fringe is also changed after completion ofall processes. When the amount of change is given to Δd, the reproducedwavelength λ′ is given by the formula (1) as follows when the hologramis reproduced.

λ′=2n(d+Δd)sin θ  (11)

[0401] Accordingly, if the thickness of the photosensitive materialafter completion of all processes is larger than the thickness at theexposure, the reproduced wavelength λ′ becomes long. On the other hand,if the thickness of the photosensitive material after completion of allprocesses is smaller than the thickness at the exposure, the reproducedwavelength λ′ becomes short.

[0402] According to the invention defined in claim 22, the peakwavelength of the hologram screen is either 525 nm or less, or 585 nm ormore, in order to avoid the peak wavelength for the relative luminosityfactor. Further, the reference light and the image light are irradiatedonto the photosensitive material to form the interference fringe in sucha way that the incident angle θ_(r) of the reference light on thephotosensitive material is different from the incident angle θ_(e) ofthe image light on the hologram screen. As a result, it is possible toreproduce the image light with superior color tone without emphasis ofthe particular wavelength of the image light.

[0403] In this case, the reproduced wavelength λc is given by thefollowing formula (5).

2nd·sin θ_(e) =λc  (12)

[0404] Where, “n” and “d” are not changed after exposure and afterproduction of the hologram element. Accordingly, when the incident angleθe of the image light onto the hologram screen is larger than theincident angle θ_(r) of the reference light, the reproduced wavelengthλc becomes shorter than the recorded wavelength. On the other hand, whenthe incident angle θe of the image light onto the hologram screen issmaller than the incident angle θr of the reference light, thereproduced wavelength λc is shifted to the wavelength which is longerthan the recorded wavelength (see FIG. 22). As a result, it is possibleto obtain the hologram element having the peak wavelength of either 525nm or less, or 585 nm or more.

[0405] According to the invention defined in claim 23, it is preferablethat an amount of angle correction, which is the difference between theincident angle θr of the reference light and the incident angle θe ofthe image light, is in the range of −5° to +5°. As a result, it ispossible to obtain the hologram screen having the superior colorreappearance. When the amount of angle correction is out of the aboverange, there is a problem in which the efficiency of the hologram isdecreased, and the image becomes dark.

[0406] According to the invention defined in claim 24, the halfbandwidth of the hologram spectrum is 100 nm or more. As a result, it ispossible to obtain the scattered and diffused output light withoutemphasis of the particular wavelength of the image light. As a result,it is possible to obtain the same effect as the case which enlarges thescattering extent of the object light, and the half bandwidth of thehologram spectrum becomes 100 nm or more.

[0407] The preferred embodiments of the third aspect of the presentinvention will be explained in detail below.

[0408] (First embodiment)

[0409] The first embodiment is explained in detail with reference toFIGS. 53 to 63. As shown in FIG. 56, the image light 331 from the imageprojector 22 is scattered and diffused on the hologram screen 31 so thatit is possible to obtain the output light 332. As a result, the image isreproduced on the hologram screen 31. In this case, the half bandwidthof the hologram spectrum is 100 nm or more on the hologram screen (seeFIG. 61).

[0410] As shown in FIGS. 53 to 55, the hologram screen 31 is formed bythe hologram element which is prepared by the interference fringe. Thatis, the diffusion light passing through the light diffusion body 3116having the large light diffusion angle is used as the object light 3117,and the non-diffusion light is used as the reference light 3118. Theobject light 3117 and the reference light 3118 are irradiated onto thephotosensitive material 3120 in order to form the interference fringe.

[0411] AS shown in FIG. 56, the display apparatus 330 is formed by thehologram screen 31, the image projector 33 and an image supply apparatus(not shown). The image projector 33 is arranged to the upper locationopposite to the observer 35. In this embodiment, the hologram element isthe transmission type, and the hologram screen is formed by the hologramelement and the transparent board bonded to the hologram element inorder to support the same.

[0412] The image light 331 from the image projector 33 is scattered anddiffused on the hologram screen 31 so that it is possible to obtain theoutput light 332. As a result, the observer 35 can observe thereproduced image on the hologram screen 31. Since the hologram elementis formed by the transparent material, it is possible to transmit theback light 333 of the background 37 which is arranged opposite to theobserver 35. Accordingly, the observer can observe the background 37with the image on the hologram screen 31. In this case, the imageprojector 33 may be arranged to the location lower than the observer 35in the opposite side of the observer 35. On the other hand, it ispossible to form the hologram screen by using the reflection typehologram element.

[0413] In the exposure optical system 310 shown in FIG. 55, the laserlight oscillated by the laser oscillator 3110 is separated into twolights by the beam splitter 3111 so as to form two separated lights 3112and 3113. The separated light 3112 becomes the divergent light afterpassing through the object lens 3122, and divergent fight becomes theparallel light 3115 after reflected by the parabolic mirror 3114. Theparallel light 3116 passes through the light diffusion body 3116 so thatit is possible to obtain the diffusion light which is used as the objectlight 3117. The other separated light 3113 becomes the divergent lightafter passing through the object lens 3121, and the divergent lightbecomes the reference light 3118. In this case, the light diffusion body3116 is formed by the double-faced ground glass having the surfaceroughness of #1000, and “#1000 means that the surface of the glass istreated by sands each having a diameter of {fraction (1/1000)} inches inorder to form the cloudy glass.

[0414] The object light 3117 and the reference light 3118 are irradiatedon the photosensitive material 3120 (i.e., the photopolymer plate madeby Dupont Co.) which is bonded on the glass plate 3123, and theinterference fringe formed by these lights is recorded on thephotosensitive material 3120. As a result, it is possible to obtain thehologram element in which the light diffusion body 3116 is recorded onthe photosensitive material 3120.

[0415] When the image light is irradiated on the hologram element, thelight diffusion body 3116 is reproduced. Accordingly, when irradiatingthe image light on the hologram element, the image light is scatteredand diffused so that the image light becomes the output light, as wellas the diffusion light output from the light diffusion body 3116.

[0416] The performance evaluation of the hologram screen according tothe third embodiment will be explained in detail below. In this case,when producing the hologram element, the number of the double-facedground glass, which forms the light diffusion body 3116, was changed inorder to change the scattering characteristic of the light diffusionbody 3116. In FIG. 57, when the number of double-faced ground glass(#1000) is changed from 1 to 4, the scattering characteristic of thelight diffusion body 3116 made by the double-faced ground glass was alsochanged.

[0417] That is, the laser light is irradiated perpendicular onto thesurface of the double-faced ground glass so as to obtain the diffusionlight, the intensity ratio of the diffusion light for each output anglewas measured. In measurement of the intensity, the intensity of thediffusion light was set to “100” when the output angle was 0° (i.e., thedirection perpendicular to the surface of the light diffusion body). Asis obvious from the drawing, when the number of the double-faced groundglass is increased, it is possible to obtain the diffusion body havingthe large scattering extent.

[0418] In this case, the light diffusion angle of the light diffusionbody means the output light in which the intensity ratio of thescattering light becomes 50. Accordingly, as shown in FIG. 57, the lightdiffusion angle of the light diffusion body formed by only onedouble-faced ground glass is 12°. Further, when two double-faced groundglasses are used, the light diffusion angle is 22°. When threedouble-faced ground glasses are used, the light diffusion angle is 36°.When four double-faced ground glasses are used, the light diffusionangle is 50°. Still further, the same measurement was performed forone-faced ground glass. As a result of measurement, the light diffusionangle was 7°.

[0419] Next, in the exposure system shown in FIG. 55, the hologramelement was produced by using one one-faced ground glass, onedouble-faced ground glass, and 2 to 4 laminated ground glasses, as thelight diffusion body 3116. The half bandwidth of the hologram spectrumof the hologram element is shown in FIG. 58. In this case, as otherconditions besides the light diffusion body 3116, the thickness of thephotosensitive material 3120 is 6 μm, the intensity ratio R/O of thereference light 3118 and the object light 3117 at the center of thephotosensitive material is 4, and the incident angle θr of the referencelight 3118 and the incident angle θe of the reproduced light on thehologram screen 31 are 30° (see FIGS. 55 and 56). As is obvious fromFIG. 58, the half bandwidth of the hologram spectrum becomes broad whenthe number of the double-faced cloudy glass is increased, i.e., when thelight diffusion angle of the light diffusion body 3116 becomes large.

[0420] The measuring method of the hologram spectrum will be explainedbelow. In FIG. 60, when measuring the hologram spectrum at the center ofthe hologram element 100, the measuring light 3501 is irradiated withthe angle θc(θc=θe) from a projector 351, and the output light 3502 isoutput from the center of the hologram element 100. In this case, alight receiver 352 is suitably moved, and the intensity of the outputlight 3502 is measured. In this case, the projector 351 corresponds tothe image projector at the display apparatus, and the light receivercorresponds to the observer. Further, it is possible to obtain the peakwavelength and the peak efficiency from the hologram spectrum shown inFIG. 61. In this case, the peak efficiency corresponds to the extremevalue of the efficiency at the hologram spectrum.

[0421] Further, in the above producing method, the hologram elementseach having the half bandwidth of 60, 80, 100, 120, 140, 160 and 180 nmwere made by suitably changing kinds, the number and other producingconditions, of the double-faced ground glass. The above hologram screenswere provided in the display apparatus, and the image light wasirradiated from the image projector in order to reproduce the image.

[0422] The image on the hologram screen was observed by sixteenobservers, and the evaluation was performed by the observers inaccordance with the following seven grades regarding the color tone ofthe image. In this case, the seven grades of the evaluation are asfollows.

[0423] GRADE 7→very good color tone, and natural color;

[0424] GRADE 6→good color tone, and no feeling of difference;

[0425] GRADE 5→slightly different color tone, but no feeling ofdifference;

[0426] GRADE 4→either of them;

[0427] GRADE 3→different color tone, and slight feeling of difference;

[0428] GRADE 2→different color tone, and feeling of difference; and

[0429] GRADE 1→entirely different color tone, and feeling of difference.

[0430] The result of the evaluation is shown in FIGS. 62 and 63. In FIG.62, the ordinate is the number of the observers, and the abscissa is thehalf bandwidth of the hologram spectrum. In FIG. 63, the ordinate is theevaluation level, and the abscissa is the half bandwidth of the hologramspectrum. As shown in FIG. 63, the grade 4 or more, i.e., the evaluationlevel having no feeling of difference, is 100 nm or more as the halfbandwidth. Further, when the half bandwidth is 140 nm or more, allobservers see the grade 5 or more. Accordingly, it is obvious that thedesirable half bandwidth is 140 nm or more.

[0431] As shown in FIG. 58, when the half bandwidth of the hologramspectrum is 100 nm or more, the light diffusion angle of the diffusionbody is approximately 9° or more. The relationship of the lightdiffusion angle between the hologram element and the light diffusionbody is shown in FIG. 59. In this case, the measuring method of thelight diffusion angle is the same method as the diffusion body. That is,in FIG. 56, the white light from the image projector 33 is irradiatedonto the hologram screen 31, and the brightness of the output light atthe center of the hologram screen 31 is set to “1”. The brightness ofthe output light was measured when the view line was swang to left andright directions. In this case, the angle which becomes a half ofbrightness of the front was set to the light diffusion angle of thehologram screen.

[0432] As shown in FIG. 59, it is obvious that the light diffusion angleof the hologram screen approximately coincides with the light diffusionangle of the light diffusion body. Accordingly, when the light diffusionangle of the hologram screen is at least 100 or more, the half bandwidthof the hologram spectrum becomes 100 nm or more so that it is possibleto obtain the color tone of the image having no feeling of difference.

[0433] The operation of the third aspect of the present invention willbe explained in detail blow. The hologram spectrum of the hologramelement is shown in FIG. 61. AS shown in FIG. 61, when the halfbandwidth becomes small, the peak of the hologram spectrum becomessharp. On the other hand, when the half bandwidth becomes large, thepeak of the hologram spectrum becomes gentle. The shape of the hologramspectrum becomes flat when the half bandwidth is large, the efficiencyfor each wavelength becomes uniform.

[0434] In the hologram screen in this embodiment, the half bandwidth ofthe hologram element is 100 nm or more. Accordingly, it is possible toobtain the scattered and diffused output light without emphasis of theparticular wavelength of the image light, and to obtain the hologramscreen which can reappear the good color tone of the image light (seeFIGS. 62 and 63).

[0435] (Second Embodiment)

[0436] The object light and the reference light are irradiated onto thephotosensitive material having the thickness of 1 to 20 μm (for example,the photopolymer made by Dupont Co. Ltd. was used.) in order to form theinterference fringe on the hologram element. The half bandwidth of thehologram element is 100 nm or more. The light diffusion body is formedby laminating three double-faced ground glasses. Further, the intensityratio R/O of the reference light and the object light at the center ofthe photosensitive material is 3, and the incident angle θr of thereference light on the photosensitive material (see FIGS. 53 and 55 inthe first embodiment) and the incident angle θe of the image light onthe hologram screen (see FIG. 56 in the first embodiment) are 40°.

[0437] The half bandwidth and peak efficiency of the hologram element,which were prepared in accordance with the above conditions, weremeasured based on the method of the first embodiment. The result ofmeasurement is shown in FIGS. 64 and 65. As shown in FIG. 64, all halfbandwidth of all hologram elements are 100 nm or more. As shown in FIG.65, when the thickness becomes thin, the peak efficiency becomes small.It is obvious that the hologram element has very good color reappearanceof the image as shown in FIGS. 62 to 65. Further, when the peakefficiency becomes small, the image becomes dark. Accordingly, when thethickness of the photosensitive material is 6 μm, it is possible toobtain the best hologram element.

[0438] (Third Embodiment)

[0439] In the third embodiment, the intensity ratio R/O of the referencelight (R) and the object light (O) is set to 0.1 to 30, and thereference light and the object light are irradiated on thephotosensitive material in order to form the interference fringe on thehologram element. In this case, the half bandwidth of the hologramspectrum is 100 nm or more. The light diffusion body is formed bylaminating four double-faced ground glasses. The intensity ratio R/O ofthe reference light and the object light is 3, and the incident angle θrand the incident angle θe are 30°. As shown in FIG. 66, when theintensity ratio R/O is in the range of 0.1 to 30, the half bandwidth is100 nm or more. It is obvious that the hologram element has the goodcolor reappearance of the image.

[0440] (Fourth Embodiment)

[0441] In this embodiment, the peak wavelength of the hologram spectrumis either 525 nm or less, or 585 nm or more. The thickness of thephotosensitive material (for example, the photopolymer made by DupontCo.) is 6 μm. The light diffusion body is formed by laminating threedouble-faced ground glasses, and the intensity ratio R/O of thereference light and the object light is 0.8, 3 and 5. Further, theincident angle θr and the incident angle θe are 30°.

[0442] As shown in FIG. 67, as well as the first embodiment, theintensity ratio R/O becomes large, the peak wavelength of the hologramis shifted to the short wavelength side. Further, the peak wavelength ofthe hologram spectrum is either 525 nm or less, or 585 nm or more. Whenthe half bandwidth is 100 nm or more, particularly, when it is 140 nm ormore, it is not necessary that the peak wavelength is 525 nm or less, or585 nm or more. When the peak wavelength is either 525 nm or less, or585 nm or more, it is possible to obtain the hologram element havinggood color reappearance.

[0443] Even if the intensity ratio R/O is in the range of 0.1 to 30,there is the case that the peak wavelength becomes out of extent. InFIG. 67, when the intensity ratio R/O is 0.8, the peak wavelength isapproximately 525 nm. When the intensity ratio is less than 0.8, thepeak wavelength becomes 525 nm or more. This is because the peakwavelength and the optimum intensity ratio R/O are changed depending onthe following conditions, i.e., the incident angle of the referencelight, the recording wavelength of the hologram element, the thicknessof the photosensitive material, and the scattering characteristic of thelight scattering body. Accordingly, although the intensity ratio R/O ischanged in order to set the peak wavelength within the above extent forevery change of conditions of the hologram production, when theintensity ratio R/O is adjusted in the range of 0.1 to 30, it ispossible to set the peak wavelength to either 525 nm or less, or 585 nmor more.

[0444] Further, there is a wavelength dependency in the sensitivity of ahuman eye. Accordingly, in the display apparatus shown in FIG. 56, animage state which was caught by the human eye is preferably reflected bythe characteristic which multiplies the hologram spectrum of the imagelight by the wavelength dependency of the human eye shown in FIG. 68.

[0445] The hologram spectrum which was multiplied by the spectralluminous efficiency is shown in FIG. 69. Since the human spectralluminous efficiency has the large peak at the wavelength of 555 nm, thelarge peak occurs in the vicinity of the wavelength of 555 nm as shownin FIG. 69.

[0446] When the image light is reproduced by using the hologram screenin which the intensity ratio R/O is 1 and the peak wavelength is 547 nmas shown in FIG. 70, the reproduced image light has the hologramspectrum which multiplies the hologram spectrum of FIG. 69 by thehologram spectrum of FIG. 70. The new hologram spectrum is shown in FIG.71. As shown in FIG. 71, the green color is emphasized in the image byoverlapping the emphasis of the green due to the hologram screen withthe emphasis of the green due to the spectral luminous efficiency.

[0447] The image is reproduced by using the hologram screen having thehologram spectrum shown in FIG. 72, and the peak wavelength of thishologram spectrum is 460 nm. The reproduced image light has the hologramspectrum which multiplies the hologram spectrum of FIG. 69 by thehologram spectrum of FIG. 72. The new hologram spectrum is shown in FIG.73. When the peak wavelength of the hologram spectrum becomes small, thepeak level of the green of the hologram spectrum is reduced as shown inFIG. 73.

[0448] Accordingly, when providing the hologram screen formed by thehologram spectrum having the peak wavelength at the wavelength of either525 nm or less, or 585 nm or more and avoiding the peak wavelength ofthe spectral luminous efficiency, it is possible to provide goodappearance of color tone of the original image without emphasis of theparticular wavelength of the image light, and to realize the hologramelement having superior color reappearance.

[0449] (Fifth Embodiment)

[0450] In this embodiment, the incident angle θr of the reference lighton the photosensitive material is different from the incident angle θeof the image light on the hologram screen, and the reference light andthe object light are irradiated on the photosensitive material in orderto form the interference fringe. The peak wavelength of the hologramspectrum is either 525 nm or less, or 585 nm or more. In this case, thethickness of the photosensitive material (for example, the photopolymermade by Dupont Co. Ltd.) is 6 μm, and the light diffusion body is formedby laminating three double-faced ground glass. Further, the intensityratio R/O of the reference light and the object light is set to 3.

[0451] The hologram screen having the angle 40° as the incident angle θeof the image light was prepared. In this case, the incident angle θr ofthe reference light was changed by 40°, 41°, 42° and 43°. That is, theamount of the angle correction (i.e., θr−θe) was determined as 0°, 1°,2° and 3°. The relationship between the peak efficiency and the amountof the angle correction is shown in FIG. 74. As shown in FIG. 74, whenthe amount of the angle correction is changed by 0°, 1°, 2° and 3°, thepeak wavelength is shifted to the short wavelength. That is, thewavelength is shifted to the short wavelength by 13 nm at +1° for 0°,and shifted to the short wavelength by 18 nm at +2° for +1°. In thiscase, when the wavelength is shifted to the long wavelength, the amountof the angle correction becomes minus, and the incident angle θr of thereference light is set so as to become smaller than the incident angleθe of the reproduced light.

[0452] As mentioned above, by changing the incident angle θr of thereference light and the incident angle θe of the reproduced light, thepeak wavelength of the hologram spectrum can be set to either 525 nm orless, or 585 nm or more. Since the hologram element according to thisembodiment is slipped from the peak of the spectral luminous efficiency,it is possible to realize good color reappearance of the image.

[0453] (Sixth Embodiment)

[0454] In this embodiment, a plurality of object lights each of whichhave different angle are irradiated onto the photosensitive material inorder to form the interference fringe. Further, the half bandwidth ofthe hologram spectrum is 100 nm or more.

[0455] As shown in FIG. 75, the separated light 3112 (see FIG. 55) isfurther separated by the beam splitter into two lights, i.e., theparallel light 3115 and the divergence light 3130. The divergence light3130 is intersected by the parallel light 3115 with the angle θ at thecenter C of the light diffusion body 3116. The parallel light 3115 andthe divergence light 3130 are irradiated onto the light diffusion body3116 so that it is possible to obtain the diffusion light which is usedas the object light 3117. On the other hand, the separated light 3113(see FIG. 55) becomes the divergence light through the object lens 3121,and the divergence light is used as the reference light 3118. Thediffusion body 3116 is formed by two laminated double-faced ground glasshaving the surface roughness of #1000.

[0456] The scattering extent of the object light was measured by thefollowing method. That is, as shown in FIG. 76, a non-transparent plate3151 having a hole 3150 at the center thereof is provided before thediffusion body 3116. The hole 3150 has a diameter φ10. The parallellight 3115 and the divergence light 3130 are irradiated to the hole3150, and output to the diffusion body 3116 with the angle θ. In thisembodiment, the intensity of the output light of the angle θ from thehole through the diffusion body 3116 was measured by using a powermeter. That is, the intensity ratio of the output light of the angle θand the output perpendicular to the diffusion body 3116 (i.e., the angle0°) was measured by the power meter.

[0457] The result of measurement is shown in FIG. 77. The curveindicated by the mark “Δ” is in the case that only parallel light 3115was irradiated onto the light diffusion body 3116, and the curveindicated by the mark “□” is in the case that the parallel light 3115and the divergence light 3130 were irradiated onto the light diffusionbody 3116.

[0458] As is obvious from the drawing, it is possible to improve thescattering extent of the object light by adding the divergence light3130 to the parallel light 3115. When the scattering extent of theobject light is increased, it is possible to produce the hologramelement having board peak wavelength so that it is possible to obtainthe hologram element having good color reappearance.

[0459] (Seventh Embodiment)

[0460] In this embodiment, a sum of the exposure intensity of theintensity of the object light and the intensity of the reference lightwas determined in the range of 0.02 to 50 mW/cm², and the object lightand the reference light were irradiated on the photosensitive materialin order to form the interference fringe on the hologram element. Thethickness of the photosensitive material, for example, photopolymer, was6 μm, and the light diffusion body was formed by three double-facedground glass. The intensity ratio of the reference light and the objectlight was 4. The incident angle θr of the reference light on thephotosensitive material and the incident angle θe of the image light onthe hologram screen were 35°. The amount of exposure was 20 mJ/cm².

[0461] As shown in FIGS. 78 and 79, when the sum of the exposureintensity (mW/cm²) becomes small, the half bandwidth becomes broad sothat the peak wavelength is shifted to the long wavelength. However, inFIG. 78, when the sum of the exposure intensity is 0.02 mW/cm² or less,the half bandwidth becomes narrow. This is because the efficiency of thehologram element was decreased due to very long exposure time. In thiscase, it is possible to prevent a narrow half bandwidth at the sum ofintensity of 0.02 mW/cm² or less, if the stability of the optical systemor the sensitivity of the photosensitive material are improved.

[0462] In this embodiment, it is possible to set the peak wavelength atthe hologram spectrum to either 525 nm or less, or 585 nm or more, andto set the half bandwidth to 100 nm or more, by adjusting the sum of theexposure intensity in the range of 0.02 to 50 mW/cm².

[0463] (Eighth Embodiment)

[0464] In this embodiment, after completion of the interference fringe,the ultraviolet light (UV light) having the intensity 0.1 mW/cm² wasirradiated onto the photosensitive material, and the heating conditionswas adjusted in the range of 80 to 150° in order to form the hologramelement. In this case, the peak wavelength is either 525 nm or less, or585 nm or more, and the half bandwidth is 100 nm or more.

[0465] Further, the thickness of the photosensitive material (thephotopolymer was used) was 6 μm and 10 μm. The light diffusion body wasformed by three laminated double-faced ground glass, and the intensityratio R/O of the reference light and the object light was 5. Further,the incident angle θr of the reference light and the incident angle θewere 30°, and the amount of exposure was 20 mJ/cm². The result ofmeasurement is shown in FIGS. 80 to 82.

[0466] As shown in FIGS. 80 and 81, when the heating temperature becomeshigh, the half bandwidth becomes broad so that the peak wavelength isshifted to the long wavelength. In FIG. 82, the heating temperature is100° C. The half bandwidth is increased during five hours, and becomesapproximately constant after five hours.

[0467] (Ninth Embodiment)

[0468] In this embodiment, after completion of the interference fringe,the hologram element was produced in accordance with the followingconditions. That is, the hologram element was irradiated by theultraviolet having the intensity of 0.1 mW/cm², and heated by 120°during one hour. As shown in FIG. 83, after above processes, thehologram element 340 was pressed by rollers 345 and 346 with a hardsubstrate 347 (for example, a glass plate, resin plate, etc.) by using alaminator 34, in the conditions of the heating temperature 140° C., thepressure of 2 kg/cm², and transfer speed of 10 cm/minute. As a result,the peak wavelength of the hologram spectrum was shifted to the longwavelength by approximately 30 nm.

[0469] In this case, since the hologram element is heated and pressed,the photopolymer becomes soft. Further, after exposure or irradiation ofthe UV, the hologram element can be pressed. In this case, it ispossible to set a low heating temperature. Further, when the dichromatedgelatin (DCG) is used as the photosensitive material, the followingadjustments are performed. That is, when the peak wavelength is shiftedto the long wavelength, a dry plate coated by the DCG before exposure,is exposed after vacuum process-in the range of 0.1 to 1 (Torr) and from10 seconds to 2 hours.

[0470] When the Ar laser having the wavelength 514 nm is used, thewavelength was shifted to the long wavelength by 550 nm in theconditions of 0.2 Torr and 10 minutes, and by 590 nm in the conditionsof 0.2 Torr and 1.5 hours. When the peak wavelength is shifted to theshort wavelength, the thickness of the photosensitive material becomesmore thick than the thickness at the exposure by performing a chemicalhard-film process after swelling of the hologram in warm water duringdevelopment. As a result, it is possible to shift the peak wavelength tothe short wavelength. In this case, after swelling of the hologram inthe warm water of 30° C. and during five minutes, and after swelling itin a rapid fixer during five minutes with 25° C., the peak wavelengthwas shifted to the 480 nm.

[0471] Explanations of the Fourth Aspect of the Present Invention

[0472] Before describing the preferred embodiments, a conventional artand its problem are explained below. There is a known transparenthologram screen which can reproduce the image based on the output lightby scattering and diffusing the image light from the image projector.The observer can observe the reproduced image on the hologram screen andcan see the background through the hologram screen. This type of thehologram screen can be utilized, for example, in the ticket window in abank or hospital, as mentioned above. Further, the hologram screen canbe utilized as an advertising board in a shopping center, and as ahead-up display apparatus.

[0473] One example of a display apparatus using the transmission typehologram screen is shown in FIG. 110. The image projector 412 isarranged at the back of the hologram screen 49, i.e., opposite to theobserver 48. The image light 4120 is irradiated from the image projector412 onto the hologram screen 49 so that the image is reproduced on thehologram screen 49 and the image can be observed by the observer 48.

[0474] On the other hand, a display apparatus using the reflection typehologram screen is shown in FIG. 111. The image projector 412 isarranged at the front of the hologram screen 490, i.e., at the same sideas the observer 48. The image light 4120 from the image projector 412 isirradiated in order to reproduce the image on the hologram screen 490.As shown in FIGS. 112 and 113, two lights, i.e., the object light 4320formed by the diffusion light through the light diffusion body 432, andthe reference light 4310 formed by non-diffusion light, are irradiatedon the photosensitive material 4310 in order to expose and form theinterference fringe thereon.

[0475] In this case, as shown in FIG. 112, when the object light 4320and the reference light 4310 are irradiated from the same side on thephotosensitive material 431, it is possible to obtain the transmissiontype hologram screen 490. On the other hand, as shown in FIG. 113, whenthe object light 4320 and the reference light 4310 are irradiated fromthe opposite side each other on the photosensitive material 431, it ispossible to obtain the reflection type hologram screen 490.

[0476] There is, however, a problem in which cloudiness occurs on thehologram screen because the interference fringe formed by the objectlight is recorded thereon. When cloudiness occurs on the screen, it isvery difficult to observe the background through the hologram screen.Particularly, when there is no image on the hologram screen, theobserver feels difference of the image at the background due to thiscloudiness.

[0477] A method of producing the hologram screen having wide view anglehas been disclosed, for example, in the Japanese Unexamined PatentPublication No. 9-127853. According to this method, the diffusion light,i.e., the object light, is obtained by irradiating a plurality oflights, each of which has different direction, onto the light diffusionbody. On the other hand, the non-diffusion light is used as thereference light. The object light and the reference light are irradiatedon the photosensitive material in order to form the interference fringe.Further, two kinds or more, each having different diffusion angle, ofthe light diffusion bodies are bonded so as to form one light diffusionbody. Then, the object light and the reference light are irradiated onthe photosensitive material in order to obtain the hologram screen.According to this method, since it is possible to use the diffusionlight in which the difference in intensity due to scattering directionis small, it is possible to obtain the hologram screen having wide viewangle.

[0478] On the other hand, since there is a proper feature in which theobserver can observe the image from only particular range (a view angle)on the hologram screen. Accordingly, the hologram screen having wideview angle has high utility in various fields. However, when the viewangle becomes broad, cloudiness is increased on the hologram screen.

[0479] As another method, a method for preventing cloudiness from theimage has been disclosed in the Japanese Patent Unexamined PublicationNo. 9-127612.

[0480] It is assumed that the reference light is not irradiated, andonly object light is irradiated on the photosensitive material to exposeit. When the efficiency η_(∞) of the hologram screen obtained by theabove conditions is 5% or less, the intensity of the object light isgiven as A. The photosensitive material is exposed by using the objectlight and the reference light having the intensity which corresponds tothe intensity A. As a result, it is possible to obtain the hologramscreen having no cloudiness on the image.

[0481] There is a problem, however, in the above method. That is, evenif the exposure is performed by the object light which exceeds theefficiency η_(∞) of 5%, there is possibility in which no cloudiness isobtained on the hologram screen. Accordingly, the above conventionalmethod has been yet insufficient in order to solve cloudiness on thehologram screen.

[0482] Further, the hologram screen using the dichromated gelatin (DCG)as the photosensitive material has been disclosed in the above JapaneseUnexamined Patent Publication No. 9-127612. In this case, it isnecessary to provide a wet development process in the producing processusing the DCG. Accordingly, when the DCG becomes hard in the developmentprocess, small cracks occur in the DCG so that the hologram screenbecomes cloudy.

[0483] The inventors found the fact that, if there is no cloudiness onthe hologram screen, the observer does not mind a cloudy state even ifthe efficiency η_(∞) becomes large. On the other hand, since thehologram screen made by the DCG frequently becomes cloudy, it isnecessary to set the efficiency η_(∞) to 5% or less.

[0484] The fourth aspect of the present invention aims to solve theabove problems, and to provide the hologram screen which has superiortransparency and no cloudiness, and can observe clearly the backgroundby the observer. Further, a producing method of the hologram screen isprovided.

[0485] According to the invention defined in claim 26, the image lightfrom the image projector is scattered and diffused to obtain the outputlight so that the image can be reproduced on the screen, and thehologram screen is characterized by a haze ratio of 5 to 60%.

[0486] As mentioned above, the hologram screen according the fourthaspect of the present invention has the haze ratio of 5 to 60%. Sincethe haze ratio is set in the range of 5 to 60%, the light which is inputfrom the external, except for the image projector, to the hologramscreen has the low scattering extent, it is possible to output the imagewith high efficiency. Accordingly, when the observer observes thehologram screen, there is no cloudiness of the hologram screen due tothe external light, and it is possible to transmit the backgroundwithout cloudiness, and to clearly observe the background through thehologram screen. That is, the observer can observe the back at the sametime the image reproduced on the hologram screen, different from thenon-transparent hologram. As a result, this transparent hologram can beutilized as, for example, the advertising board in the shopping center.Further, the transparent hologram screen can be utilized as a head-mountdisplay of a mobile system.

[0487] When the haze ratio exceeds 60%, there is cloudiness on thehologram screen so that it is difficult to clearly observe the back. Onthe other hand, when the haze ratio is 5% or less, the efficiency of thehologram screen becomes low so that the image becomes dark.

[0488] In general, a haze ratio which indicates cloudiness of atransparent plate has been defined, for example, in the JIS-K7105(Japanese Industrial Standard), and this is used in the presentinvention. In the case of the hologram screen, even if the haze ratiobecomes high, the observer can clearly observe the back, i.e., thehologram screen has high transparency.

[0489] The haze ratio can be expressed as follows.

H=Td/Tt×100  (13)

[0490] Where, H is a haze ratio (%), Td is a diffused transmission (%),and Tt is an all lights transmission (%). Concretely, Tt is the lightintensity which is transmitted perpendicularly to the center of thelight diffusion body or the hologram screen (precisely, the lightintensity of the scattering light in the range of 3.5°), and Td is thetotal intensity of the scattering light at all directions except for thecenter direction.

[0491] The cause of cloudiness of the hologram screen will be explainedcompared to cloudiness of the light diffusion body.

[0492] In the general light diffusion body, the incident light isscattered due to unevenness of the surface, or due to particles existingwithin the diffusion body and scattering the light. As shown in FIG.103, the beam of light 471 from the object 470 at the back of the lightscattering body 472 to the observer 48 passes through the lightscattering body 472 and output to the observer 48. In this case, a partof beam of light 473 goes straight to the observer 48, and the remaininglights 4731 and 4732 are always bended to another directions which aredifferent from the direction of the observer 48. From this reason, thereis obscuration of the image of the object 470 at the back when theobserver 48 observes the image.

[0493] When the haze ratio becomes small, the bending extent of thelights 4731 and 4732 becomes small. However, except that the bendingextent is extremely small, the obscuration of the object 470 alwaysoccurs when the observer 48 observes the image. For example, when thebeam of lights 4731 and 4732 are bent by 0.1°, there is the differenceof 3.5 mm at the observer location which is apart from the lightscattering body 472 by the distance of 2 m. Further, there is thedifference of 8.7 mm at the observer location which is apart from thelight scattering body 472 by the distance of 5 m. In general, since adiameter of the pupil of human eye is approximately 5 mm, the beam oflights 4731 and 4732 are bent by only 0.1°, the obscuration of theobject 470 occurs at the location of the observer 48.

[0494] On the other hand, the hologram screen, as mentioned below, thediffusion body which is formed by transmitting the laser light throughthe light diffusion body or reflecting the laser light therefrom is usedas the object light, and the non-diffusion light is used as thereference light. The interference fringe is formed by the object lightand the reference light, and recorded as the difference in therefractive index of the material of the photosensitive material. Therecorded interference fringe functions as the diffractive lattice. Ingeneral, the material of the hologram screen is transparent, and thesurface thereof is flat.

[0495] As shown in FIG. 104, the direction of the beam of light 471which directs from the object 470 to the observer 48, is not changed bythe interference fringe 4100 recorded on the hologram screen 41.Accordingly, the beam of light 471 becomes the beam of light 474directing to the observer 48 through the hologram screen 41, and theobserver can observe the object 470 without obscuration. As a result,the hologram screen 41 appears to be transparent by the observer 48.

[0496] In this case, a part of the beam of light 471 is diffracted bythe interference fringe 4100 and output from the hologram screen 41 asthe beam of light 475. Since the incident direction of the beam of light474 is different from the incident direction of the image light, theintensity of the beam of light 475 becomes low compared to the beam oflight 474. However, since there are many interference fringes 4100, thebeam of light 475 may become the light having a certain grade of theintensity. When the intensity of the beam of light 475 becomes strongdue to efficiency and number of the interference fringe 4100, the hazeratio of the hologram screen 41 becomes high.

[0497] However, the beam of light 474 which is not diffracted (see FIG.104) goes straight to the observer 48, and the light path of the beam oflight 474 is not bent. Accordingly, it is possible to preventobscuration of the object 470 which is arranged at the back of thehologram screen 41. In the case of the hologram screen 41, even if ithas high haze ratio compared to the normal light scattering body, it ispossible to see the hologram screen as it is transparent.

[0498] On the other hand, cloudiness which-causes non-transparency ofthe hologram screen, occurs due to the fact that the external light isdiffracted by the normal interference fringe (i.e., interference fringeformed by the object light and reference light) and another interferencefringe formed by two object lights (i.e., unnecessary interferencefringe), and the diffracted light is output to the direction of theobserver's view line. In this case, the external light indicates thelights which are input to the hologram screen, except for the imagelight irradiated to the hologram screen and the object light from theback of the hologram screen.

[0499]FIG. 105 is a schematic view of the interference fringe 100recorded on the hologram screen 41. In FIG. 105, the interference fringeis shown by three lines each having different slant. However, inactuality, one line is formed by a plurality of parallel fringes so thatthere are many lines each having different angles.

[0500] In the interference fringe 100, the interference fringe 101 isrecorded by the reference light 4310 and the object light 4321, theinterference fringe 102 is recorded by the reference light 4310 and theobject light 4322, and the interference fringe 103 is recorded by thereference light 4310 and the object light 4323, as shown in FIG. 106.Accordingly, in the interference fringe 100, the image light which wasinput from the same direction as the reference light 4310 can beeffectively diffracted, but the lights which are input from otherdirections cannot be effectively diffracted. Further, as shown in FIG.105, when the external lights 4711, 4712 and 4713 are input to thehologram screen 41, a part of the external light is diffracted to thedirection of the observer's view line.

[0501] As shown in FIG. 107, the interference fringe 4109 is normallyformed by the reference light 4310 and the object lights 4328 and 4329,and, at the same time, it is also formed by the object lights 4328 and4329. In this case, the interference fringe formed by-two object lightsis called “Fresnel noise”, and a part of the external light isdiffracted due to “Fresnel noise”.

[0502] As mentioned above, when the external light is diffracted by thenormal interference fringe and another interference fringe due to“Fresnel noise”, and output to the direction of the observer's viewline, the observer feels cloudiness of the hologram screen. That is, thecloudiness of the hologram screen is caused by a cloudy state of thewhole of the surface of the hologram screen when the external light isdiffracted by the interference fringe and output to the direction of theobserver's view line. That is, cloudiness is not caused by thescattering of the background light. Even if the scattering extent of thebackground light lies in a certain high grade, i.e., even if the hazeratio is high, cloudiness of the hologram is not observed by theobserver when the intensity of the external light which is diffracted tothe observer is weak. As mentioned above, in the case of cloudiness ofthe hologram screen, even if the haze ratio is high, the cloudinessextent which is observed by the observer is small and different fromcloudiness of the normal light scattering body.

[0503] The result of comparison of the hologram screen and the normallight scattering body is explained below. The scattering characteristic(i.e., an intensity distribution for each output angle of the outputlight) of the transmission light of the hologram screen having the hazeratio of 30% and the light diffusion body having the haze ratio of 5%(for example, an anti-glare film AG-30 made by NITTO DENKO Co., Ltd.) isshown in FIG. 108. The hologram screen used in this embodiment was madebased on a method explained in the following second embodiment.

[0504] As shown in FIG. 108, the light scattering body has the hazeratio of 5% and the intensity of the output light at the direction ofthe output angle of 0° is approximately 95%. On the other hand, thehologram screen has the haze ratio of 30%, and the intensity of theoutput light at the direction of the output angle of 0° is approximately75%. That is, in the hologram screen, it is possible to output the beamof light having the intensity of 75%, which is not scattered. Since thelight having this intensity is not scattered, it is possible torecognize the back of the hologram screen. When the haze ratio is 50%,the light can be transmitted by approximately 65% so that it is possibleto sufficiently observe the back of the hologram screen. However, sincethe intensity of the transmission light becomes low, the back becomesslightly dark. According to the present invention, it is possible toprovide the hologram screen which has good transparency and nocloudiness, and can be observed clearly at the back.

[0505] According to the invention defined in claim 27, it is preferablethat a screen gain of the hologram screen is 0.3 or more. As a result,it is possible to obtain the hologram screen in which the brightness ofthe image on the hologram screen is not reduced. Accordingly, it ispossible to obtain the hologram screen having the clear and superiorimage.

[0506] When the hologram screen is arranged into a popular room, theback brightness of the hologram screen becomes 400 cd/m². Accordingly,it is necessary to provide the brightness exceeding the above in thebrightness of the image which is imaged on the hologram screen (see FIG.85).

[0507] On the other hand, a high brightness type image projector (forexample, a liquid crystal projector) which is available in the markethas a minimum projecting display size of 40 inches. Further, it has beenknown that the maximum illuminance of the image projected on thisdisplay size is approximately 4000 lux. Further, in general, the screengain can be obtained based on the following formula.

(screen gain)=(brightness×n)/illuminance

[0508] Accordingly, the screen gain which becomes the illuminance of4000 (lux) and the brightness of 400 (cd/m²) is approximately 0.3. Asmentioned above, it is necessary to set the screen gain to 0.3 or morein order not to reduce the brightness of the image on the hologramscreen. If the screen gain is 0.3 or less, the image on the hologramscreen becomes dark, and it may be difficult to distinguish from theback of the hologram screen. In this case, the upper limit of the screengain must be determined so as not to exceed the haze ratio of 60% sincethe haze ratio is increased in accordance with increase of the screengain. Accordingly, the screen gain cannot be determined uniformly sinceit is dependent on the characteristic of the hologram screen.

[0509] According to the invention defined in claim 28, in thetransmission type or reflection type hologram screen, the intensityratio E_(R)/E_(O) of the intensity E_(O) of the object light and theintensity E_(R) of the reference light is changed in accordance with thescattering angle of the light diffusion body. In this case, thescattering angle of the light diffusion body is an angle determined whenthe intensity ratio Iθ/I_(O) is 0.5. Where, the intensity I_(O) isdefined by the light intensity which is output perpendicularly (i.e.,direction of angle 0°) from the light diffusion body, and the intensityIθ is defined by the light intensity which is output to the direction ofangle 0°) from the light diffusion body.

[0510] As shown in FIG. 109, the intensity E_(O) of the object light4320 and the intensity E_(R) Of the reference light 4310 are given by asum of intensity of the object light 4310 and the reference light 4320which are irradiated on a unit area of the photosensitive material fromall directions. Even if the intensity ratio E_(R)/E_(O) is constant,since the scattering angle of the light diffusion body is changed, theintensity Iθ_(O) of the object light 4325 which is output to a certaindirection, i.e., direction of the angle θ, becomes weak. When theintensity 100 becomes weak, the efficiency η_(RO) of the interferencefringe becomes low.

[0511] Accordingly, when the scattering angle of the light diffusionbody is changed, it is necessary to change the intensity 100 in ordernot to decrease the efficiency η_(RO) In this case, to change theintensity 100 means the change of the intensity E_(O), i.e., theintensity ratio E_(R)/E_(O). If the efficiency η_(RO) becomes low, thereduced image becomes dark.

[0512] Further, even if the intensity ratio E_(R)/E_(O) is constant, theefficiency of the “Fresnel noise” which is formed by the interferencefringe between two object lights becomes large when the scattering angleof the light diffusion body becomes large. As mentioned above, when theefficiency of the “Fresnel noise” becomes large, the cloudiness of thehologram screen becomes large so that the it becomes the non-transparentstate. Accordingly, it is necessary to change the intensity ratioE_(R)/E_(O) in order to decrease the efficiency of the “Fresnel noise”.

[0513] As mentioned above, by changing the intensity ratio E_(R)/E_(O)in accordance with a size of the scattering angle of the light diffusionbody, it is possible to increase the efficiency η_(RO) and to decreasethe efficiency of the “Fresnel noise”. In the hologram screen in whichthe efficiency η_(RO) is higher and the efficiency of the “Fresnelnoise” is lower, the incident light to the hologram screen is notscattered so that it is possible to irradiate the output light with highefficiency. That is, it is possible to easily produce the hologramscreen having the haze ratio of 5 to 60%.

[0514] When the haze ratio is 5 to 60%, it is possible to irradiate theimage light with high efficiency. In this case, the scattering extent ofthe light, which is input from other light sources except for the imageprojector to the hologram screen, becomes low. Accordingly, when theobserver observes the hologram screen, the cloudiness of the hologramscreen due to the external light is small, the light from the back ofthe hologram screen is transmitted without cloudiness, and can reach theobserver. Accordingly, the observer can clearly observe the back throughthe hologram screen. That is, according to the fourth aspect of thepresent invention, it is possible to produce the transparent hologramscreen without cloudiness.

[0515] In the exposure optical system, as a method for changing thescattering angle of the light diffusion body, there are two methods,i.e., a method for exchanging the light diffusion body, and a method forusing the light diffusion body integrated by laminating a plurality oflight scattering bodies each having the same scattering angle or eachhaving different scattering angle.

[0516] Further, the intensity ratio E_(O)/E_(R) by suitably changing theintensities E_(O) and E_(R). For example, it is possible to adjust thechange of the intensities E_(O) and E_(R) by suitably changing thereflection factor of each mirror and magnification of the object lens.In particular, it is preferable to change the reflection factor of thehalf-mirror since it is possible to avoid loss of the laser light.

[0517] In the producing method in this embodiment, it is possible toutilize a photopolymer and dichromated gelatin as the photosensitivematerial. However, there is the following problem. That is, since it isnecessary to provide a wet type development process in the producingprocess using the dichromated gelatin, there is a problem in whichcracks occur on the photosensitive material during producing process sothat cloudiness occurs on the hologram screen. In the present invention,however, it is possible to provide the hologram screen withoutcloudiness by using various photosensitive materials, and to obtain thetransparent hologram screen.

[0518] According to the invention defined in claim 29, when enlargingthe scattering angle of the light diffusion body, it is preferable toreduce the above intensity ratio E_(R)/E_(O). As a result, it ispossible to obtain the hologram screen without cloudiness since the hazeratio becomes 5 to 60% or less.

[0519] Further, it is possible to obtain the diffusion light havingbroad extent by enlarging the scattering angle of the light diffusionbody, and to also obtain the hologram screen having broad view field byusing the diffusion light as the object light. In the hologram screenhaving broad view field, since the image can be observed without anarrow limitation of the observer position, this hologram screen can beutilized without limitation in the place in actual use.

[0520] The embodiment will be explained with reference to FIGS. 85 to 87below. As shown in FIG. 85, the image light 4120 from the imageprojector 412 is scattered and diffused so that the output light 4121can be obtained. Further, the hologram screen has the haze ratio of 5 to60%.

[0521] As shown in FIG. 85, the hologram screen 41 is arranged in ashowroom 42. In the showroom 42, an exhibition 4210 is mounted on a wall421, and the hologram screen 41 is mounted on an inside of a windowglass 420. The observer 48, who positioned at the outside of theshowroom 42, can observe the image on the hologram screen 42. Further, alamp 423 is mounted on a ceiling 424 of the showroom 42, and the lightfrom the lamp 423 becomes a part of the output light passing through thehologram screen 41 and is sent to the observer 48. The light from thelamp 423 is irradiated on the exhibition 4210, and reflected thereby.Further, the light reflected by the exhibition is-sent to the observer48. A liquid crystal projector is used as the image projector 412 whichis mounted on the ceiling 424.

[0522] In this embodiment, the hologram screen 41 is the transmissiontype, and a method of producing the hologram screen was explained in thesecond embodiment. On the other hand, as shown in FIG. 111, it ispossible to arrange the image projector 412 at the same side of theobserver 48 by using the reflection type hologram screen. Further, it ispossible to arrange the image projector 412 on the floor 425.

[0523] Next, in the hologram screen according to the embodiment, anevaluation test was performed regarding the relationship between thehaze ratio and the cloudiness which was observed by the observer asfollows. That is, in an environment shown in FIG. 85, the evaluationtest was performed by twenty observers regarding visual recognition ofthe exhibition 4210 and grade of cloudiness of the hologram screen.

[0524] As a method for obtaining the hologram screen having differenthaze ratio which was used in this evaluation test, the method forproducing the hologram screen shown in the embodiment was utilized. Thatis, the intensity ratio E_(R)/E_(O) of the reference light and theobject light is changed in accordance with the scattering angle of thelight diffusion body in the exposure optical system shown in FIG. 88.One hologram screen was formed by laminating a plurality of hologramscreens in order to obtain the desired haze ratio.

[0525] The hologram screen 41 used in the evaluation test has the widthacross corners of 20 inches. The horizontal illuminance of the lightfrom the lamp 423 was 1000 (lux), and the brightness on the wall 421which becomes the background of the hologram screen 41 was 400 (cd/m²).Further, the distance between the wall 421 and hologram screen 41 was 5m, and the illuminance at the side of the observer 48 was 1000 (lux).Since the average illuminance is 500 (lux) in an office room, and asmall shop, such as a convenience store, is 700 (lux), the brightness ofthe lamp 423 was set to 1000 (lux).

[0526] The visual recognition of the back of the hologram screen 41 wasevaluated by the following test. That is, Gothic black charactersincluding Chinese characters and alphabet, and each having a size of36-point, were drawn on a white paper. This white paper was mounted onthe wall 421 as the background of the hologram screen and as theexhibition 4210. This exhibition 4210, which is not irradiated by theimage projector 412, was observed by the observer 48 who positionedapart from the hologram screen 41 by two meters. Then, the state of thecharacters (i.e., whether it is easy for the observer to read characterson the white paper) and the cloudiness extent of the back due to thehologram screen, were subjectively evaluated by the observers.

[0527] The result of the evaluation is shown in FIGS. 86 and 87. In FIG.86, the evaluation scale was determined as follows. That is, GRADE 7denotes “very easy to read”, GRADE 6 denotes “easy to read”, GRADE 5denotes “slightly easy to read”, GRADE 4 denotes “either of them”, GRADE3 denotes “slightly difficult to read”, GRADE 2 denotes “difficult toread”, and GRADE 1 denotes “cannot read”.

[0528] On the other hand, in FIG. 87, the evaluation scale wasdetermined as follows. That is, GRADE 7 denotes “completelytransparent”, GRADE 6 denotes “almost transparent”, GRADE 5 denotes“slightly cloudy, but don't mind”, GRADE 4 denotes “slightly cloudy, buteither of them”, GRADE 3 denotes “cloudy, and do mind”, GRADE 2 denotes“cloudy, and do mind strongly”, and GRADE 1 denotes “completelynon-transparent”.

[0529] As shown in FIG. 86, when the haze ratio of the hologram screen41 is 60% or less, all twenty observers judged as “no problem to readcharacters” (i.e., all observers lie in GRADE 4 or more). Further, asshown in FIG. 87, when the haze ratio is 60% or less, all observersobserve slightly cloudy state, but don't mind feeling of difference(i.e., all observers lie in GRADE 4 or more). As mentioned above, whenthe haze ratio is 60% or less, it is possible to clearly observe theback through the hologram screen, and to obtain the image withoutcloudiness.

[0530] (Second Embodiment)

[0531] The method for producing the hologram screen is explained withreference to FIGS. 88 to 102. As shown in FIG. 88, the object light 4320and the reference light 4310 are irradiated on the photosensitivematerial 431 in order to form (record) the interference fringe thereon.When changing the scattering angle of the light diffusion body 432, theintensity ratio E_(R)/E_(O) of the intensity E_(O) of the object light4320 and the intensity E_(R) of the reference light 4310 is changed.

[0532] In FIG. 88, the exposure optical system 43 is provided forproducing the hologram screen. In this structure, the adjustment of theintensity ratio E_(R)/E_(O) of the intensity E_(O) of the object light4320 and the intensity E_(R) of the reference light 4310 was performedby suitably changing the transmission factor of the half mirror 4391,the reflection factor of each mirror 4392, 4393, 4395 and 4396, and themagnification of the object lens 4394 and 4397.

[0533] In this case, there is the relationship of L/S=3/2 between thedistance L (i.e., a length of light diffusion body 432) and the distanceS (i.e., a distance between the light diffusion body 432 and thephotosensitive material 431). Further, the photopolymer made by DupontCo., Ltd. having the thickness of 6 μm was used as the photosensitivematerial 431, and the double-faced ground glass having the surfaceroughness of #1000 was used as the light diffusion body 432.

[0534] As mentioned above, the object light 4320 and the reference light4310 are irradiated onto the photosensitive material 431 in order torecord the interference fringe thereon. An amount of the exposure on thephotosensitive material 431 was 30 mJ/cm². After completion of theinterference fringe, the ultraviolet was irradiated on thephotosensitive material 431 with the intensity of 0.1 mJ/cm², and thephotosensitive material 431 was heated by temperature 120° C. during twohours, so that the hologram screen was obtained.

[0535] Since the photosensitive material 431 is formed by a thin film,when it is used as the hologram screen, the thin film is supported by atransparent plate which is bonded thereto. Various performances of theabove hologram screen were evaluated by the following test. In thistest, the light diffusion body was formed by four kinds of lightdiffusion bodies each having different angle as shown in FIG. 89. Thehaze ratio of each hologram screen was measured as shown in FIG. 90. Asshown in FIG. 90, when the intensity E_(O)/E_(R) is the same, the hazeratio becomes small in accordance with increase of the scattering angleof the light diffusion body. On the other hand, when the haze ratio isthe same, the intensity E_(O)/E_(R) becomes small in accordance withincrease of the scattering angle of the light diffusion body. As isobvious from the graph, when the intensity E_(O)/E_(R) becomes large,the above characteristic appears considerably. In FIG. 91, therelationship between the scattering angle and the intensity E_(O)/E_(R)in the case of the haze ratio of 30% and 50% is shown in FIG. 91.

[0536] Further, the relationship between the haze ratio and theintensity E_(R)/E_(O) due to the “Fresnel noise” formed by theinterference fringe between two object lights, was checked as follows.That is, in the exposure optical system 43 shown in FIG. 88, the lightdiffusion body having the scattering angle of 36° was used, and the hazeratio due to the “Fresnel noise” was checked for the case only when theintensity E_(R)/E_(O) is 20 or less, as shown in FIG. 93.

[0537] As shown in FIG. 93, when the intensity E_(R)/E_(O) is 6 or less,the haze ratio is considerably increased. When the intensity E_(R)/E_(O)is 10 or more, the haze ratio becomes constant by approximately 3%.Accordingly, the haze ratio of 3% is considered as the haze ratio basedon the material itself which forms the hologram screen.

[0538] Further, as is obvious from comparison of the graph of FIG. 90with the graph of FIG. 92, the hologram screen having the interferencefringe due to the “Fresnel noise” has the lower haze ratio. This reasonis as follows. That is, when the photosensitive material is exposed byonly object light, all energy of the object light are used for thepreparation of “Fresnel noise”. However, when there is the referencelight, an amount of energy to be consumed for the “Fresnel noise”becomes small. Further, the haze ratio based on the normal interferencefringe formed by the object light and the reference light becomes low.On the other hand, the haze ratio which occurs in the large intensityE_(R)/E_(O) is almost caused by the normal haze ratio.

[0539] Accordingly, when producing the hologram screen having relativelyhigh haze ratio, if the haze ratio due to the “Fresnel noise” is checkedas shown in FIGS. 92 and 93, and if the intensity of the object lightand the amount of exposure are set to the same as the desired hazeratio, it is possible to set the haze ratio of the hologram screen to avalue lower than the desired haze ratio.

[0540] AS shown in FIG. 92, in the case of the hologram screen producedbased on the “Fresnel noise” and the scattering angle of 12° as thelight diffusion body, the haze ratio becomes 5% when the intensityE_(R)/E_(O) is 6. For the hologram screen having the haze ratio of 5%, a“completely transparent” hologram screen was recognized by all twentyobservers (i.e., This corresponds to GRADE 7 of the evaluation).

[0541] On the other hand, as shown in FIG. 90, in the case of thehologram screen produced based on the normal interference fringe and thescattering angle of 12° as the light diffusion body, the haze ratiobecomes 20% when the intensity E_(R)/E_(O) is 6. For the hologram screenhaving the haze ratio of 20%, a “transparent” hologram screen wasrecognized by all twenty observers (i.e., This corresponds to GRADE 6and GRADE 7).

[0542] When the hologram screen is produced based on the aboveprocesses, it is possible to reduce the haze ratio due to the “Fresnelnoise” until the grade in which there is no influence for cloudiness ofthe hologram screen. Further, it is possible to reduce the haze ratiodue to the normal interference fringe until the grade in which thetransparency of the hologram screen can be ensured.

[0543] Next, in the exposure optical system shown in FIG. 88, therelationship between the scattering angle of the light diffusion bodyused at the production of the hologram screen and the intensityE_(R)/E_(O) at the exposure is shown in FIG. 94. Since the haze ratio ofthe hologram screen is approximately 5%, it is very transparent (seeFIGS. 86 and 87). When the scattering angle becomes large, it ispossible to obtain the hologram screen having the same haze ratio byreducing the intensity E_(R)/E_(O).

[0544] Further, the relationship between the diffraction efficiencyη_(RO) of the normal interference fringe at the hologram screen, whichwas produced by using the light diffusion body of the scattering angleof 36°, and the intensity E_(R)/E_(O) at the exposure, as shown in FIG.95. In the measurement of the diffraction efficiency η_(RO), as shown inFIG. 96, the light 4751 was irradiated with the angle θc to the hologramscreen 41, and the light 4752, which was transmitted through thehologram screen 41 without diffraction by the interference fringe, wasmeasured. The distribution of the transmitted light 4752 is shown inFIG. 97, and an area shown by slant lines is defined as the diffractionefficiency η_(RO) of the hologram screen.

[0545] As shown in FIG. 95, the diffraction efficiency η_(RO) becomeslarge in accordance with decrease of the intensity E_(R)/E_(O). When theintensity E_(R)/E_(O) is 3 or less, it becomes approximately constant.Accordingly, as shown in FIGS. 90 and 95, when producing the hologramscreen, by producing it when the intensity E_(R)/E_(O) is 3, it ispossible to reduce the haze ratio of the hologram screen withoutdecrease of the normal diffraction efficiency η_(RO) Further, since thehologram screen has the haze ratio of 30%, cloudiness of the hologramscreen was not recognized by the observers (see FIGS. 86 and 87).

[0546] Further, the intensity E_(R)/E_(O), is obtained for each lightdiffusion body shown in FIG. 89, and the relationship between theintensity and the scattering angle is shown in FIG. 98. As shown in FIG.98, it is necessary to reduce the intensity E_(R)/E_(O) in accordancewith increase of the scattering angle of the light diffusion body. Thescreen gain of the hologram screen shown in FIG. 98 is shown in FIG. 99.As shown in FIG. 99, the screen gain at each scattering angle exceeds0.3.

[0547] As shown in FIGS. 90 and 95, the haze ration becomes high inaccordance with increase of efficiency of the hologram screen. Further,as shown in FIG. 92, the haze ratio due to the “Fresnel noise” alsobecomes high.

[0548] As is obvious from the above explanations, in the case that thebrightness of the image is not always necessary, i.e., the efficiency ofthe hologram screen is not always required based on an environmentalconditions using the hologram screen, it is possible to produce thehologram screen having good transparency by reducing the diffractionefficiency η_(RO) of the hologram screen.

[0549] Further, in the change of the scattering angle of the lightdiffusion body 432, the scattering angle is adjusted in such a way that,the larger the scattering angle, the smaller the intensity ratioE_(R)/E_(O), when changing the intensity ratio E_(R)/E_(O). As a result,it is possible to clearly observe the back through the hologram screen,and to produce the hologram screen having no cloudiness and goodtransparency. Further, when the screen gain is 0.3 or more, it ispossible to produce the hologram screen which can clearly recognize theimage.

[0550] Even if the exposure conditions is set as the diffractionefficiency η_(RO) of the normal interference fringe of the hologramscreen is constant, there is the case in which the haze ratio becomeslow in accordance with increase of the efficiency of the Fresnel noise”.For example, in the production of the hologram screen based on theconditions in which the scattering angle of the light diffusion body is36° and intensity ratio E_(R)/E_(O) is 3, as shown in FIG. 100, when theamount of exposure becomes constant or more, the diffraction efficiencyη_(RO) of the normal interference fringe becomes constant, but the hazeratio is increased as shown in FIG. 101. In this case, the amount ofexposure is determined to a value in which the diffraction efficiencyη_(RO) becomes constant, i.e., 30 mJ/cm².

[0551] Further, it is possible to change the haze ratio in accordancewith the thickness of the photosensitive material when producing thehologram screen. The change of the haze ratio is shown in FIG. 102 whenchanging the thickness of the photosensitive material. As is obviousfrom FIG. 102, it is preferable to use thin film as far as possible inthe extent of characteristic of the hologram screen in order to obtainlow haze ratio.

[0552] As shown in FIG. 112, the object light and the reference lightare irradiated from the same direction. In this case, it is possible toobtain the transmission type hologram screen. On the other hand, theobject light and the reference light are irradiated from the differentdirection each other. In this-case, it is possible to obtain thereflection type hologram screen.

1. A hologram screen for reproducing an image based on an output lightobtained by scattering and diffusing an image light from an imageprojector, characterized in that; when a white light is used as theimage light and projected on the hologram screen; and when a distancebetween two points at optional two points A and B on a surface of thehologram screen is given by 20 cm or less, and a value of the CIE 1976chromaticity coordinate (u′, v′) at the point A is given by (U′_(A),V′_(A)) and the value of the CIE 1976 chromaticity coordinate (u′, v′)at the point B is given by (u′_(B), v′_(B)); the output light, which isoutput perpendicularly from the surface of the hologram screen, has acolor distribution in which a color difference Δu′v′ between two pointsA and B is derived from the following formula (1) and given by 0.06 orless, where, the formula (1) is expressed by, Δu′v′=[(u′ _(A) −u′_(B))²+(v′ _(A) −v′ _(B))²]^(1/2)  (1)
 2. A hologram screen forreproducing an image based on an output light obtained by scattering anddiffusing an image light from an image projector, characterized in that;when a white light is used as the image light and projected on thehologram screen; and when a distance between two points at optional twopoints A and B on a surface of the hologram screen is given by 20 cm orless, and a value of the CIE 1976 chromaticity coordinate (u′, v′) atthe point A is given by (u′_(A), v′_(A)) and the value of the CIE 1976chromaticity coordinate (u′, v′) at the point B is given by(u′_(B)/v′_(B)); the output light, which is output to a viewpointdefined by the following formula (2) from the surface of the hologramscreen, has a color distribution in which a color difference Δu′v′between two points A and B is derived from the following formula (1) andgiven by 0.01 or less, where, the formula (1) is expressed by,Δu′v′=[(u′ _(A) −u′ _(B))²+(v′ _(A) −v′ _(B))²]_(1/2)  (1) and, where,the formula (2) is expressed by, H/2L=0.1  (2) in the formula (2), L isa distance between the viewpoint and a center of the hologram screen,and H is a length of the hologram screen at the direction of the height.3. A hologram screen as claimed in claim 1, wherein a spectralcharacteristic of the output light, which is output perpendicularly fromthe surface of the hologram screen, has a characteristic in which eithera difference between peak wavelengths at the whole of the surface of thehologram screen is given by 120 nm or less, or a difference between halfbandwidth is given by 100 nm or less.
 4. A method for producing ahologram screen in an exposure optical system in which a diffusion lightobtained by a light diffusion body is used as an object light, and anon-diffusion light is used as a reference light, characterized in that,when producing the hologram screen defined in claim 1 or 2 byirradiating the object light and the reference light onto aphotosensitive material and by exposing the photosensitive material, thephotosensitive material has a thickness distribution which is inclinedto an incident direction of the reference light.
 5. A method forproducing a hologram screen as claimed in claim 4, wherein when anincident angle of the reference light to the photosensitive materialbecomes large, the thickness distribution of the photosensitive materialbecomes thick.
 6. A method for producing a hologram screen as claimed inclaim 4, wherein when the thickness at a center of the photosensitivematerial is T₀, and when thickness difference of both ends of thephotosensitive material is ΔT, the following relationship, i.e., ΔT≦0.5T₀, is defined.
 7. A method for producing a hologram screen as claimedin claim 4, wherein the thickness distribution of the photosensitivematerial is defined by the following formula (3), i.e., T=a(R _(O) −R)+T₀, and a=b·θ ₀ ^(−0.9)  (3) where, T is a thickness at an optional pointon the photosensitive material, T₀ is a thickness at the center of thephotosensitive material, R is an incident distance of the referencelight at the optional point on the photosensitive material, R₀ is theincident distance of the reference light at the center on thephotosensitive material, θ₀ is an incident angle of the reference lightat the center of the photosensitive material, and “b” is a coefficientdetermined by the thickness of the photosensitive material and given by0<b<1.
 8. A method for producing a hologram screen in an exposureoptical system in which a diffusion light obtained by a light diffusionbody is used as an object light, and a non-diffusion light is used as areference light, characterized in that, when producing the hologramscreen defined in claim 1 or 2 by irradiating the object light and thereference light onto a photosensitive material and by exposing thephotosensitive material, an ultraviolet is previously irradiated ontothe photosensitive material so as to have an energy distribution whichis inclined to an incident direction of the reference light.
 9. A methodfor producing a hologram screen as claimed in claim 8, wherein theenergy distribution of the ultraviolet is given by the following formula(4), i.e., 0.8E≦Euv<1.2E, and E=0.01(R−R _(O))+E ₀  (4) where, R is theincident distance of the reference light at the optional point on thephotosensitive material, Euv is an amount of energy of the ultravioletirradiated to the optional point on the photosensitive material, R₀ isthe incident distance of the reference light at the center of thephotosensitive material, and E₀ is the amount of energy of theultraviolet irradiated to the center of the photosensitive material. 10.A method for producing a hologram screen in an exposure optical systemin which a diffusion light obtained by a light diffusion body is used asan object light, and a non-diffusion light is used as a reference light,characterized in that, when producing the hologram screen defined inclaim 1 or 2 by irradiating the object light and the reference lightonto a photosensitive material and by exposing the photosensitivematerial, an incident distance of the reference light at an optionalpoint of the reference light is shorter than an incident distance of animage light which is irradiated from an image projector when reproducingthe image on the hologram screen.
 11. A method for producing a hologramscreen in an exposure optical system in which a diffusion light obtainedby a light diffusion body is used as an object light, and anon-diffusion light is used as a reference light, characterized in that,when producing the hologram screen defined in claim 1 or 2 byirradiating the object light and the reference light onto aphotosensitive material and by exposing the photosensitive material, anintensity ratio of the reference light and the object light is definedby a relationship of I_(R)/I_(O)<10, on the whole of the surface of thephotosensitive material, where, I_(R) is the intensity of the referencelight, and I_(O) is the intensity of the object light.
 12. A method forproducing a hologram screen in an exposure optical system in which adiffusion light obtained by a light diffusion body is used as an objectlight, and a non-diffusion light is used as a reference light,characterized in that, when producing the hologram screen defined inclaim 1 or 2 by irradiating the object light and the reference lightonto a photosensitive material and by exposing the photosensitivematerial, at least two sets of the following steps a) to d) are combinedwhen producing the hologram screen, i.e., a) step of using thephotosensitive material having the thickness distribution which isinclined to an incident direction of the reference light; b) step ofusing the photosensitive material previously irradiated by anultraviolet so as to have an energy distribution which is inclined tothe incident direction of the reference light; c) step of shortening anincident distance of the reference light at an optional point on thephotosensitive material than the incident distance of the image lightwhich is irradiated from an image projector when reproducing the imageon the hologram screen; and d) step of defining an intensity ratio ofthe reference light and the object light by the relationship ofI_(R)/I_(O)≦10, on the whole of the surface of the photosensitivematerial, where, I_(R) is the intensity of the reference light, andI_(O) is the intensity of the object light.
 13. A method for producing ahologram screen formed by interfering of a reference light and an objectlight transmitted through a light diffusion body, and by recording thelight diffusion body on a photosensitive material, characterized inthat; a mirror is arranged approximately perpendicular to the lightdiffusion body; the photosensitive material and the light diffusion bodyare rotated by the same angle θ around an axis which is intersectedperpendicularly to a center of the photosensitive material and used as arotational center; and a polarized direction of a laser light is definedby either P-polarization or S-polarization which is inclined by theangle in the range of θ−5 to θ+5.
 14. A hologram screen for reproducingan image based on an output light obtained by scattering and diffusingan image light from an image projector, characterized in that; a halfbandwidth of a spectral characteristic of the hologram screen is givenby 100 nm or more; a diffusion light which is obtained by a lightdiffusion body having a large light diffusion angle is used as an objectlight; a non-diffusion light is used as a reference light; and theobject light and the reference light are irradiated on a photosensitivematerial in order to form an interference fringe so that a hologramelement is produced.
 15. A hologram screen as claimed in claim 14,wherein the light diffusion angle of the-hologram screen is defined byan angle in which the light diffusion angle of the hologram screen is10° or more.
 16. A hologram screen for reproducing an image based on anoutput light obtained by scattering and diffusing an image light from animage projector, characterized in that; a half bandwidth of a spectralcharacteristic of the hologram screen is given by 100 nm or more; adiffusion light which is obtained by a light diffusion body is used asan object light; a non-diffusion light is used as a reference light; andthe object light and the reference light are irradiated on aphotosensitive material having thickness of 1 to 20 μm in order to forman interference fringe so that a hologram element is produced.
 17. Ahologram screen for reproducing an image based on an output lightobtained by scattering and diffusing an image light from an imageprojector, characterized in that; a peak wavelength of the hologramscreen is either 525 nm or less, or 585 nm or more; a half bandwidth ofa spectral characteristic of the hologram screen is 100 nm or more; adiffusion light which is obtained by a light diffusion body is used asan object light; a non-diffusion light is used as a reference light; theobject light and the reference light are irradiated on a photosensitivematerial in order to form an interference fringe; and a refractive indexof the photosensitive material is adjusted so that a hologram element isproduced.
 18. A hologram screen as claimed in claim 17, wherein thediffusion light which is obtained by the light diffusion body is used asthe object light; the non-diffusion light is used as the referencelight; the object light and the reference light are irradiated on thephotosensitive material in order to form the interference fringe; and aheat treatment is performed for the photosensitive material in the rangeof 800 to 150° C. so that a hologram element is produced.
 19. A hologramscreen as claimed in claim 17, wherein the diffusion light which isobtained by the light diffusion body is used as the object light; thenon-diffusion light is used as the reference light; a sum of an exposureintensity of the object light and the exposure intensity of thereference light is defined in the extent of 0.02 to 50 mW/cm²; theobject light and the reference light are irradiated on thephotosensitive material in order to form the interference fringe so thata hologram element is produced.
 20. A hologram screen as claimed inclaim 17, wherein the diffusion light which is obtained by the lightdiffusion body is used as the object light; the non-diffusion light isused as the reference light; an intensity ratio (R/O) of the intensity(O) of the object light and the intensity (R) of the reference right isdefined by 0.1 to 30; and the object light and the reference light areirradiated on the photosensitive material in order to form theinterference fringe so that a hologram element is produced.
 21. Ahologram screen for reproducing an image based on an output lightobtained by scattering and diffusing an image light from an imageprojector, characterized in that; a peak wavelength of the hologramscreen is either 525 nm or less, or 585 nm or more; a diffusion lightwhich is obtained by a light diffusion body is used as an object light;a non-diffusion light is used as a reference light; the object light andthe reference light are irradiated on a photosensitive material in orderto form an interference fringe; and a thickness of of the photosensitivematerial is adjusted so that a hologram element is produced.
 22. Ahologram screen for reproducing an image based on an output lightobtained by scattering and diffusing an image light from an imageprojector, characterized in that; a peak wavelength of the hologramscreen is either 525 nm or less, or 585-nm or more; a diffusion lightwhich is obtained by a light diffusion body is used as an object light;a non-diffusion light is used as a reference light; an incident angle θrof the reference light to a photosensitive material is different from anincident angle θe of the image light to the hologram screen; and theobject light and the reference light each having the above differentangle are irradiated on the photosensitive material in order to form aninterference fringe so that a hologram element is produced.
 23. Ahologram screen as claimed in claim 22, wherein an amount of anglecorrection which indicates a difference between the incident angle orand the incident angle θe is defined by the extent of −5° to +5°.
 24. Ahologram screen for reproducing an image based on an output lightobtained by scattering and diffusing an image light from an imageprojector, characterized in that; a half bandwidth of a spectralcharacteristic of the hologram screen is given by 100 nm or more; adiffusion light which is obtained by a light diffusion body is used asan object light; a non-diffusion light is used as a reference light; anda plurality of object lights each having different angle are irradiatedon a photosensitive material in order to form an interference fringe sothat a hologram element is produced.
 25. A hologram screen forreproducing an image based on an output light obtained by scattering anddiffusing an image light from an image projector, characterized in that;when producing the hologram screen based on the conditions in which ahalf bandwidth of a spectral characteristic of the hologram screen isgiven by 100 nm or more, and a peak wavelength of the hologram screen iseither 525 nm or less, or 585 nm or more; any one or more of thefollowing steps (A) to (H) is used when producing the hologram screen,i.e., (A) step of forming the interference fringe on a photosensitivematerial in such a way that a diffusion light which is obtained by alight diffusion body having a large diffusion angle is used as an objectlight, a non-diffusion light is used as a reference light, and theobject light and the reference light are irradiated on thephotosensitive material; (B) step of forming the interference fringe onthe photosensitive material in such a way that the diffusion light whichis obtained by the light diffusion body is used as the object light, thenon-diffusion light is used as the reference light, and the object lightand the reference light are irradiated on the photosensitive materialhaving a thickness of 1 to 20 μm; (C) step of forming the interferencefringe on the photosensitive material in such a way that the diffusionlight which is obtained by the light diffusion body is used as theobject light, the non-diffusion light is used as the reference light, anintensity ratio (R/O) of an intensity (O) of the object light and theintensity (R) of the reference light is given by 0.1 to 30; and theobject light and the reference light are irradiated on thephotosensitive material; (D) step of forming the interference fringe onthe photosensitive material in such a way that the diffusion light whichis obtained by the light diffusion body is used as the object light, thenon-diffusion light is used as the reference light, an incident angle θrof the reference light to the photosensitive material is different froman incident angle θe of the image light to the hologram screen; and theobject light and the reference light each having the above differentangle are irradiated on the photosensitive material in order to form theinterference; (E) step of forming the interference fringe on thephotosensitive material in such a way that the diffusion light which isobtained by the light diffusion body is used as the object light, thenon-diffusion light is used as the reference light, and a plurality ofobject lights each having different angle are irradiated on thephotosensitive material in order to form the interference fringethereon; (F) step of forming the interference fringe on thephotosensitive material in such a way that the diffusion light which isobtained by the light diffusion body is used as the object light, thenon-diffusion light is used as the reference light, the object light andthe reference light are irradiated on the photosensitive material inorder to form the interference fringe, and a heat treatment is performedin the extent of 80° to 150° C.; (G) step of forming the interferencefringe on the photosensitive material in such a way that the diffusionlight which is obtained by the light diffusion body is used as theobject light, the non-diffusion light is used as the reference light, asum of an exposure intensity of the exposure intensity of the objectlight and the exposure intensity of the reference light is given by theextent of 0.02 to 50 mW/cm², and the object light and the referencelight are irradiated on the photosensitive material in order to form theinterference fringe; and (H) step of forming the interference fringe onthe photosensitive material in such a way that the diffusion light whichis obtained by the light diffusion body is used as the object light, thenon-diffusion light is used as the reference light, the object light andthe reference light are irradiated on the photosensitive material inorder to form the interference fringe, and the thickness of thephotosensitive material is adjusted.
 26. A hologram screen forreproducing an image based on an output light obtained by scattering anddiffusing an image light from an image projector, characterized in that,a haze ratio is given by 5 to 60%.
 27. A hologram screen as claimed inclaim 26, wherein a screen gain of the hologram screen is given by 0.3or more.
 28. A method for producing a hologram screen in which an objectlight which is obtained by transmitting a light diffusion body or byreflecting on the light diffusion body, and a reference light obtainedby a non-diffusion light, are irradiated on a photosensitive material toform an interference fringe; and the interference fringe is recorded onthe photosensitive material; an intensity ratio E_(R)/E_(O) of theintensity E_(O) of the object light and the intensity E_(R) of thereference light is changed in accordance with a scattering angle of thelight diffusion body.
 29. A method for producing a hologram screen asclaimed in claim 28 wherein, when the scattering angle of the lightdiffusion body is set to a large angle, the intensity ratio E_(R)/E_(O)is set to a small value.